Mickey C. Webb appeals the district court’s summary judgment affirmance of the Commissioner of Social Security’s final decision denying him disability insurance benefits under Title II of the Social Security Act, 42 U.S.C. §§ 401-33. On appeal, Webb contends that the administrative law judge (“ALJ”) improperly found him to be not disabled at the second step of the five-step evaluative process, and that substantial evidence did not support the ALJ’s finding that Webb’s impairments were not medically severe. We agree and reverse.
I.
In 1987, at the age of 35, Webb had his torso partially crushed in an all-terrain vehicle accident. He suffered several broken ribs and internal injuries and was hospitalized for a week. Two years later, he collapsed at his job as a retail manager and was taken to the emergency room, where he was found to have elevated blood pressure. In 1991, Webb experienced visual disturbances and memory loss; he underwent a CAT scan, but its results were normal. Then, in 1994, he began to have acute back pain that his doctor traced to the ATV accident and a workplace injury in 1982 or 1983.
During the next several years, Webb continued to suffer from lower back pain and hypertension. He went on and off a variety of medications. Although some had positive results, their side effects were often intolerable. Because of his ailments and the side effects of their treatment, Webb stopped working.
Although there are gaps in Webb’s treatment history, the record before the ALJ included doctors’ reports and other medical evidence documenting his problems. X-rays taken in 1994 showed “disc space narrowing” in his lower back. A doctor’s report stated that “[degenerative disc disease [wa]s suspected.” In 1995, after attempting to help a friend build a garage door, Webb experienced severe hip pain; a clinical report indicated that he endured similar hardship prior to that “after getting in and out of small cars.” In 1996, he also began to have knee pain. Doctors’ reports from that year reported “tissue swelling” in Webb’s left knee and “some degenerative changes medially and some minimal patellar spurring.” Throughout this period, Webb’s back problems appear to have been constant. His other conditions grew better or became worse in correspondence with his ability to endure the side effects of his therapeutic regimen.
In 2000, Webb filed an application for disability insurance benefits in which he alleged disability since 1991 due to back pain, high blood pressure, memory loss, arm pain, weakness and lack of sleep. Although Webb presented evidence of his problems after 1997, his insurance coverage expired in that year. The relevant period for the purpose of assessing his disability status, therefore, is prior to 1997. In 2002, after a hearing, the ALJ found Webb not to be disabled. When the Appeals Council denied Webb’s request for review, the ALJ’s decision became the final agency decision for purposes of this appeal. Pursuant to 42 U.S.C. § 405(g), Webb sought judicial review of the Commissioner’s final decision in the district court, which affirmed the ALJ. He appeals that judgment. We have jurisdiction to hear Webb’s appeal under 28 U.S.C. § 1291.
II.
We review the district court’s order affirming the Commissioner’s denial of *686benefits de- novo. Smolen v. Chater, 80 F.3d 1273, 1279 (9th Cir.1996). We uphold the Commissioner’s decision denying benefits if the Commissioner applied the proper legal standard and there is substantial evidence in the record as a whole to support the decision. Hoffman v. Heckler, 785 F.2d 1423, 1425 (9th Cir.1986). Substantial evidence is such relevant evidence as a reasonable mind might accept as adequate to support a conclusion. Richardson v. Perales, 402 U.S. 389, 401, 91 S.Ct. 1420, 28 L.Ed.2d 842 (1971).
III.
A. Relevant Legal Framework
The Social Security Act defines disability as “the inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than 12 months.” 42 U.S.C. § 423(d)(1)(A). A claimant bears the burden of establishing disability under the Act. Tackett v. Apfel, 180 F.3d 1094, 1098 (9th Cir.1999). To determine whether a claimant has established disability, an ALJ must evaluate the evidence adduced by following a five-step, sequential analysis. 20 C.F.R. § 404.1520. At step one, the ALJ examines whether the claimant is engaged in substantial gainful employment activity. Id. § 404.1520(a)(4)(i). At step two, the ALJ assesses whether the claimant has a medically severe impairment or combination of impairments that significantly limits his ability to do basic work activities. Id. § 404.1520(a)(4)(h). The “ability to do basic work activities” is defined as “the abilities and aptitudes necessary to do most jobs.” 20 C.F.R. § 404.1521(b). For purposes of Webb’s petition, the most relevant activities include the ability to perform “physical functions such as walking, sitting, lifting, pushing, pulling, reaching, carrying, or handling.” Id. An impairment is not severe if it is merely “a slight abnormality (or combination of slight abnormalities) that has no more than a minimal effect on the ability to do basic work activities.” S.S.R. No. 96-3(p) (1996). If the ALJ finds that the claimant lacks a medically severe impairment, the ALJ must find the claimant not to be disabled. However, if the ALJ concludes that the claimant does have a medically severe impairment, the ALJ proceeds to the next steps in the sequence.
Steps three through five require the ALJ to evaluate whether the claimant’s impairment satisfies certain statutory requirements entitling him to a disability finding. If the impairment does not, the ALJ must assess whether the claimant remains capable of doing his prior work or engaging in alternative employment. 20 C.F.R. § 404.1520(a)(4)(iii)-(v).
B. Findings by the ALJ
In assessing Webb’s petition, the ALJ first determined that Webb was not engaged in any gainful employment activity. At step two, the ALJ found that Webb did not have a medically severe impairment or combination of impairments prior to the lapse in his insurance coverage in 1997. Having made that finding, the ALJ ended his inquiry because there was no way that Webb could prove he was disabled within the meaning of the Act. It is this preemptive finding that Webb challenges.
An impairment or combination of impairments may be found “not severe only if the evidence establishes a slight abnormality that has no more than a minimal effect on an individual’s ability to work.” Smolen, 80 F.3d at 1290 (internal quotation marks omitted) (emphasis add*687ed); see Yuckert v. Bowen, 841 F.2d 303, 306 (9th Cir.1988). The Commissioner has stated that “[i]f an adjudicator is unable to determine clearly the effect of an impairment or combination of impairments on the individual's ability to do basic work activities, the sequential evaluation should not end with the not severe evaluation step.” S.S.R. No. 85-28 (1985). Step two, then, is “a de minimis screening device [used] to dispose of groundless claims,” Smolen, 80 F.3d at 1290, and an ALJ may find that a claimant lacks a medically severe impairment or combination of impairments only when his conclusion is “clearly established by medical evidence.” S.S.R. 85-28. Thus, applying our normal standard of review to the requirements of step two, we must determine whether the ALJ had substantial evidence to find that the medical evidence clearly established that Webb did not have a medically severe impairment or combination of impairments. See also Yuckert, 841 F.2d at 306 (“Despite the deference usually accorded to the Secretary’s application of regulations, numerous appellate courts have imposed a narrow construction upon the severity regulation applied here.”).
In this case, the ALJ found that Webb lacked a medically severe impairment or combination of impairments despite objective medical evidence demonstrating back pain, hypertension, knee pain, hip pain, visual disturbances, memory loss, diverticulitis, lack of sleep, difficulty performing physical tasks and lack of employment from 1991 through 1997. Although the medical record paints an incomplete picture of Webb’s overall health during the relevant period, it includes evidence of problems sufficient to pass the de minimis threshold of step two. See Smolen, 80 F.3d at 1290. And although Webb ultimately bears the burden of establishing his disability, see Bowen, 482 U.S. at 146, 107 S.Ct. 2287, the ALJ had an affirmative duty to supplement Webb’s medical record, to the extent it was incomplete, before rejecting Webb’s petition at so' early a stage in the analysis. See 20 C.F.R. § 404.1512(e)(1); S.S.R. 96-5p (1996). “In Social Security cases the ALJ has a special duty to fully and fairly develop the record and to assure that the claimant’s interests are considered.” Brown v. Heckler, 713 F.2d 441, 443 (9th Cir.1983) (per curiam). The ALJ’s duty to supplement a claimant’s record is triggered by ambiguous evidence, the ALJ’s own finding that the record is inadequate or the ALJ’s reliance on an expert’s conclusion that the evidence is ambiguous. See Tonapetyan v. Halter, 242 F.3d 1144, 1150 (9th Cir.2001). Here, the medical evidence was sufficiently ambiguous to trigger the ALJ’s duty because of the obvious vicissitudes in Webb’s health, particularly the ways in which his conditions improved and worsened as a result of the afflictions and their treatments.
Moreover, on the record that does exist, the ALJ’s reasons for rejecting Webb’s complaints at step two are not substantial enough to meet the “clear and convincing” standard when balanced against Webb’s doctors’ contemporaneous observations, some objective tests and Webb’s subjective complaints. See Reddick v. Chater, 157 F.3d 715, 722 (9th Cir.1998) (“Unless there is affirmative evidence showing that the claimant is malingering, the Commissioner’s reasons for rejecting the claimant’s testimony must be clear and convincing.”) (internal citation and quotation marks omitted). The ALJ found that Webb’s “subjective complaints” and “assertions regarding the disabling extent of his functional limitations ... [we]re exaggerated, and not credible” because Webb was not under constant treatment, Webb was capable of performing house*688hold tasks and Webb sought employment during the relevant period. However, the medical record indicates that Webb’s effort to have his hypertension treated often resulted in new afflictions and complications due to the deleterious side effects of his medication. The record also includes x-rays showing “disc space narrowing” in his lower back and his doctor’s opinion that Webb may have suffered from “degenerative” back conditions and “disc fragmentation or significant herniation.” That Webb sought employment suggests no more than that he was doing his utmost, in spite of his health, to support himself.1 “The mere fact that a plaintiff has carried on certain daily activities, such as grocery shopping, driving a car, or limited walking for exercise, does not in any way detract from [his] credibility as to [his] overall disability. One does not need to be ‘utterly incapacitated’ in order to be disabled.” Vertigan v. Halter, 260 F.3d 1044, 1050 (9th Cir. 2001) (internal citation omitted).
The ALJ also viewed Webb’s objective medical evidence simply as part of “the claimant’s subjective complaints” when finding Webb’s assertions to be “exaggerated, and not credible.” Credibility determinations do bear on evaluations of medical evidence when an ALJ is presented with conflicting medical opinions or inconsistency between a claimant’s subjective complaints and his diagnosed conditions. See, e.g., Batson v. Comm’r of Soc. Sec. Adm’n, 359 F.3d 1190, 1195 (9th Cir.2004). But there is no inconsistency between Webb’s complaints and his doctors’ diagnoses sufficient to doom his claim as groundless under the de minimis standard of step two. Webb’s clinical records did not merely record the complaints he made to his physicians, nor did his physicians dismiss Webb’s complaints as altogether unfounded. To the contrary, the doctors’ reports and test results usually corresponded with the afflictions Webb perceived, particularly his hypertension, the side effects of his treatment and the interaction of some drugs (his pain killers) with others (his hypertension medication) in his therapeutic regimen. There is not, in this instance, the total absence of objective evidence of severe medical impairment such as was the case in Ukolov v. Barnhart, 420 F.3d 1002 (9th Cir.2005), where we affirmed a finding of no disability at step two when even the claimant’s doctor was hesitant to conclude that any of the claimant’s symptoms and complaints were medically legitimate. Id. at 1006.
We do not intimate that Webb will succeed in proving that he is disabled and entitled to disability insurance benefits. But we do hold that the ALJ lacked substantial evidence to find that the medical evidence clearly established Webb’s lack of a medically severe impairment or combination of impairments. The ALJ should have continued the sequential analysis beyond step two because there was not substantial evidence to show that Webb’s claim was “groundless.” See Smolen, 80 F.3d at 1290.
REVERSED and REMANDED.
. The ALJ determined that the record depicted "a pattern of only transitory and intermittent medical conditions rather than any chronic condition which would produce significant work-related restrictions.” Webb might not have suffered from a single, chronic condition during the relevant period, but he has submitted evidence of a combination of impairments that, on this record, appears to have prevented him from engaging in gainful activity on a sustained basis. See 20 C.F.R. § 404.1512(a). In Gatliff v. Com. of Soc. Sec., 172 F.3d 690, 691 (9th Cir.1999), we held that "[s]ubstantial gainful employment cannot be pieced together from a collection of insubstantial attempts” like Webb's vain efforts between 1991 and 1997.