Walter Morgan appeals from the district court’s decision upholding the determination of the Secretary of Health and Human Services (“Secretary”) that he is not entitled to disability insurance benefits. We affirm.
Morgan was born October 29, 1924, and partially completed the third-grade. His relevant work experience has been as a truck driver and logger, but he has not been gainfully employed since 1977. On August 22, 1984 he applied for disability and Supplemental Security Income (“SSI”) insurance benefits, alleging back and heart ailments. The application was denied initially and on reconsideration. Morgan filed for SSI and disability benefits again on August 22, 1984, which were also denied. Morgan appealed. On June 11, 1985, an Administrative Law Judge (“AU”) found Morgan eligible for SSI benefits as of July 25, 1984, because of subsequent mental impairments, but denied the disability benefits on the grounds that Morgan was not disabled on or before December 31, 1979, the date he was last insured for disability benefit purposes. Morgan appealed the AU’s determination.
The Appeals Council remanded the case for reconsideration under the Reform Act of 1984. On remand, a different AU came to the same decision as the first AU. Morgan appealed this decision. The Appeals Council affirmed the award of SSI benefits beginning July 25, 1984 but issued its own decision denying the disability claim. The Appeals Council found the conclusions of Morgan’s treating physicians that he was disabled to be inconsistent with the medical evidence, and it made the following findings:
The claimant had the following impairment on and prior to December 31, 1979: status post bilateral hernia repair, but did not have an impairment or combination of impairments listed in or medically equal to one listed in Appendix 1, Sub-part P, Regulations No. 4, at any time on or before December 31, 1979.
The claimant had no medically determinable mental impairment at any time on or before December 31, 1979, when he last met the special earnings requirements. The claimant’s subjective complaints of pain in his chest, heart, right knee and both hips are credible only to the extent that his history of bilateral hernia repair would have prevented him from performing greater than medium work at any time on or before December 31, 1979.
On the basis of these findings the Appeals Council determined that Morgan was not disabled prior to December 31, 1979, his last insured date, because he was capable of performing his past relevant work as a truck driver. Morgan appeals.
In order to obtain disability benefits, Morgan must demonstrate that he was disabled prior to his last insured date. See 42 U.S.C. § 423(c); 20 C.F.R. § 404.1520. Morgan’s last insured date was December 31, 1979. The burden of proof on this issue *1428is on the claimant. See Gamer v. Secretary of Health and Human Services, 815 F.2d 1275, 1278 (9th Cir.1987). The Secretary’s decision to deny benefits “ ‘will be disturbed only if it is not supported by substantial evidence or it is based on legal error.’ ” Brawner v. Secretary of Health and Human Services, 839 F.2d 432, 433 (9th Cir.1988) (quoting Green v. Heckler, 803 F.2d 528, 529 (9th Cir.1986)).
Morgan claims to have suffered a broken spine in 1960, and he apparently underwent a laminectomy at that time. On December 15, 1977, he was hospitalized for repair of bilateral inguinal hernias. He was discharged on December 22, 1977, after an “unremarkable” stay with a diagnosis of bilateral inguinal hernias (repaired) and acute lumbosacral strain, which was the result of a fall suffered two days prior to admission. Morgan’s treating physician, Dr. George Kaspar, stated on an insurance form on January 16, 1978, that Morgan would be disabled until February 10, 1978. A subsequent “certificate of attending physician” indicated that Morgan was released “to perform regular duties” on February 1, 1978. On March 13, 1978, Dr. Kaspar diagnosed Morgan as having parosysmal tachycardia (rapid pulse).
Morgan was hospitalized from December 1, 1978 to December 6, 1978 for chest pain. His heart was monitored continuously, and no acute abnormalities were detected. Dr. Kaspar’s discharge diagnosis, dated January 14, 1979, stated “cardiac disease to be ruled out.” Subsequent medical examinations were similarly unable to discern any significant cardiac abnormality, and one report concluded that Morgan’s condition was “probably psychogenic.” 1 However, in an insurance form dated February 20, 1979, Dr. Kaspar claimed that Morgan was totally and indefinitely disabled by his heart condition and hernias. Dr. Kaspar gave no reason for his changed opinion and provided no clinical basis to support this opinion. This opinion was inconsistent with the clinical and laboratory findings in the record, and the courts below properly disregarded it. See Coats v. Heckler, 733 F.2d 1338, 1340 & n. 4 (9th Cir.1984) (weight to be accorded to doctor’s statement depends on extent to which it is supported by clinical findings). Morgan’s heart problems did not support an inference of disability prior to December 31, 1979.
Morgan’s back problems also do not indicate the existence of a compensable disability prior to the last insured date. On December 20, 1977 Morgan was examined and found to have degenerative arthritis, but no acute skeletal abnormality. Although Morgan was observed to have ambulatory difficulty when he was hospitalized in 1977 for his hernias, he had recently suffered a fall, and residual functional capacity assessment forms in 1984 indicated that Morgan was physically “unlimited.” No medical evidence has been offered that would indicate that Morgan was precluded from engaging in his work as a truck driver prior to December 31, 1979.
Similarly, no evidence has been offered that would support an inference that Morgan became mentally disabled on or before his last insured date, December 31, 1979. The significant date for disability compensation is the date of onset of the disability rather than the date of diagnosis. See Swanson v. Secretary of Health and Human Services, 763 F.2d 1061, 1065 (9th Cir.1985); Social Security Rule (“SSR”) 83-20, Policy Statement (“The onset date of disability is the first day an individual is disabled as defined in the Act and the regulations.”). Mental disorders may manifest themselves over a period of time. Consequently, the precise date of onset of a disabling psychological impairment may be difficult, or impossible, to ascertain, and the services of a specialist may in some situations be necessary to infer the onset date.
There are no indications that prior to December 31, 1979, Morgan’s mental condition was disabling, and there is nothing in the record supporting the conclusion that *1429Morgan had any mental impairments prior to 1980. Morgan dates the onset of his disability to 1977 when he underwent his hernia operation. Although his present physical disability may well have been triggered by the hernia operation, the first evidence in the record of a mental impairment occurred in January 1980 when Morgan was treated at a clinic for anxiety and referred to a counselor. In March 1980 a rheumatologist characterized Morgan as “nearly incapacitated by severe depression and chronic anxiety,” but concluded that Morgan’s pending divorce would be “salutary” to his mental health. On December 9, 1981, a consultive examination revealed a “not quite appropriate response,” and concluded that any limitation on Morgan’s work ability would be associated with anxiety of a hypochondriacal nature. Finally, examinations performed in September and October 1984 revealed possible schizophrenia, which was confirmed in 1985.
The dissent points to the “significant record evidence documenting his [Morgan’s] increasing mental decompensation around December 1979,” but we are unable to find anything on the record that would support this conclusion. The record reveals only a progressive deterioration of his mental condition from a state of anxiety in 1980 to full-blown schizophrenia in 1985. However, this does not lead inevitably to the conclusion that he was mentally disabled in 1979.
The ALJ did not request the services of a medical examiner to evaluate Morgan’s mental disability claim. Social Security Ruling 83-20 (SSR 83-20) does not require an AU to call on the services of a medical advisor in every instance in which the onset date of a disability must be inferred. The relevant language of SSR 83-20 provides:
In some cases, it may be possible, based on the medical evidence to reasonably infer that the onset of a disabling impairments) occurred some time prior to the date of the first recorded medical examination, e.g., the date the claimant stopped working. How long the disease may be determined to have existed at a disabling level of severity depends on the informed judgment of the facts in the particular case. This judgment, however, must have a legitimate medical basis. At the hearing, the- administrative law judge (AU) should call on the services of a medical advisor when onset must be inferred. If there is information in the file indicating that additional medical evidence concerning onset is available, such evidence should be secured before inferences are made.
The Rule indicates that any inference of disability must have a legitimate medical basis. When no legitimate medical basis can support an inference of disability, no medical advisor is necessary. While Morgan did not suddenly become schizophrenic in 1984, there is no legitimate medical basis to support the inference that Morgan was schizophrenic more than five years prior to his psychiatric diagnosis of schizophrenia.
We realize that the development and manifestation of mental impairment tends to be unpredictable and that inferences must often be made; consequently, it is not possible to articulate a general rule as to when medical assistance will be necessary to infer an onset date. We hold only that in this case the record evidence was insufficient to support an inference that Morgan was mentally disabled on or before December 31, 1979, and that the AU did not err in failing to appoint a medical examiner to infer the onset date.
We likewise find no error in the development of the administrative record. Morgan was represented by competent counsel, and the record was more than adequate to enable the AU to make an informed decision regarding Morgan’s eligibility for disability benefits. This is not a case, such as those cited by the dissent, in which the claimant was not represented by counsel, or where the lapse of time was relatively short, or where the record was seriously lacking in critical evidence. There are no unexplained gaps in the record that would constitute grounds for a remand. This is also not a situation in which the medical evidence indicates that the claimant’s mental disability commenced *1430on or about the date of a traumatic injury. See Lichter v. Bowen, 814 F.2d 430, 436 (7th Cir.1987) (medical report opined that claimant suffered serious psychological damage in automobile accident).
CONCLUSION
Even though Morgan claims to have suffered from a mental impairment on or before December 31, 1979, his last insured date, the first evidence of a mental impairment in the record was on January 30, 1980, when he was treated at a medical clinic for “extreme anxiety.” The evidence on the record, which was adequately developed, was insufficient to support a reasonable inference that Morgan was mentally or physically disabled on or before December 31, 1979.
AFFIRMED.
. A consultive examination performed on December 9, 1989 revealed a slight dilation in the right ventricle and mild arteriosclerotic heart disease with possible early angina. [Record 274]