Reynolds brought a claim for workers’ compensation benefits against his employer, Reynolds Construction Company, claiming that job-related mental worry and fatigue caused him to suffer a disabling stroke. The administrative law judge’s (ALJ) award of benefits to Reynolds was adopted and affirmed by the appellate division of the State Board of Workers’ Compensation, and the superior court entered an order affirming the appellate division. We granted the employer’s application for a discretionary appeal.
Reynolds, a 37-year-old man, worked for a construction business owned by his family. The symptoms of the stroke first became apparent one afternoon as he returned home from viewing a potential job site. Testimony received from Reynolds and his brother, who also worked for the family’s construction business, indicates that, for approximately three years prior to the stroke, Reynolds was subject to extreme fatigue and stress associated with running the business. Reynolds and his brother testified that this work-related fatigue and stress caused or contributed to Reynolds’ stroke. A review of the rec*24ord reflects that the level of fatigue and stress Reynolds experienced as a result of his job was indeed extreme.1
No medical expert testified at the hearing about Reynolds’ stroke; however, certain medical records were introduced including a letter from Reynolds’ treating physician. As to the cause of the stroke, the physician’s letter merely stated: “The stroke Mr. Reynolds suffered is not directly related to any work related injury, but is simply the result of arteriosclerosis.” The physician’s letter does not address the role stress or fatigue may have played in causing Reynolds’ stroke or aggravating his condition. See Griggs v. Lumbermen’s Mut. Cas. Co., 61 Ga. App. 448 (6 SE2d 180) (1939) aff’d 190 Ga. 277 (9 SE2d 84) (1940) (employer assumes risk that job requirement may aggravate employee’s pre-existing disease or condition). Reynolds, acting pro se, did not offer any expert medical evidence. The ALJ determined, using “natural inferences” and his own “human experience” as well as the testimony of Reynolds and his brother, that Reynolds’ stroke was a work-related injury.
On appeal, the employer asserts that, in determining the cause of an injury attributed to work-related fatigue or stress, a factfinder cannot rely on inferences drawn from his own experience or on the observations of credible lay witnesses if there is conflicting expert medical evidence as to the injury’s cause.
While we agree with the employer’s position that injuries allegedly caused by work-related fatigue and stress should be closely scrutinized, the issue of whether these injuries should be compensable is not before us. It has already been decided. Ga. Bureau of Investigation v. Worthington, 149 Ga. App. 628 (255 SE2d 99) (1979) (stroke-like injury precipitated by stress is compensable). Our role is simply to determine if any evidence supports the ALJ’s determination. If so, it is our duty to affirm. Guye v. Home Indem. Co., 241 Ga. 213, 215 (244 SE2d 864) (1978).
Georgia law recognizes three forms of competent evidence for establishing a causal connection between work activities and cardiovascular problems such as heart attack and stroke: (1) medical opinion, (2) lay observation and opinion and (3) the “natural inference through human experience.” Hiers and Potter, Ga. Workers’ Compensation — Law and Practice, § 6-2 citing Guye, supra, and Carter v. Kansas City Fire &c. Ins. Co., 138 Ga. App. 601 (226 SE2d 755) *25(1976). The position advanced by the employer would carve out an exception to the general rule with regard to stress- and fatigue-related injuries. It is our opinion that such an exception is not desirable nor is it permissible under Georgia law.
As Presiding Judge McMurray previously recognized: “Neither law nor medicine are exact sciences, and we have not reached that degree of certainty where a medical expert can state a definite and certain diagnosis that the claimant’s [injury that was allegedly stress-related] did not arise out of and in the course of his employment.” Zippy Mart v. Fender, 170 Ga. App. 617, 619 (317 SE2d 575) (1984) (injury precipitated by job-related stress was compensable). If we recognize that medical science cannot always provide a definite answer as to the role stress or fatigue may have caused in an injury, then we certainly cannot permit medical evidence alone to be considered conclusive on that issue in this matter.
The present case aptly demonstrates the dangers in allowing such weight to be attributed to medical evidence. The medical records in question offered only a cursory one-sentence pronouncement as to causation and did not clearly address Reynolds’ claim that his job-related stress and fatigue caused, contributed to or aggravated his injury.2 Using the employer’s reasoning, even though the medical evidence was scanty at best, the trier of fact would be required to accept the same and ignore the testimony of two credible lay witnesses as well as his own experience. This would also be the case where the trier of fact did not believe the expert’s testimony.
Longstanding Georgia law provides factfinders freedom to accept or reject evidence as they see fit, particularly expert opinion evidence. “ ‘[Expert] testimony is not conclusive or controlling and is submitted for whatever the [factfinder] considers it to be worth. The [factfinder] can consider such expert opinion testimony by reference to their own experience and may discard the opinion of experts entirely.’ [Cit.]” (Emphasis supplied.) Wilson v. Professional Ins. Corp., 151 Ga. App. 712, 713 (261 SE2d 450) (1979). The exception which the employer seeks to create interferes with the rights, duties and obligations of the trier of fact and would result in a judicial determination of the credibility of evidence. If that is to be the state of the law, then there is no reason to have a trier of fact, as they would be bound to accept as true that which they do not believe.
In light of the above, we hold that the ALJ’s “natural inference” did not disappear and the testimony of credible lay witnesses was not rendered meaningless simply because limited medical evidence was *26offered that supported a different conclusion as to an injury’s cause. The right of the trier of fact to assess the credibility of evidence should not be so limited or second-guessed, and this court is not free to substitute its judgment for that of the ALJ in this matter. Hughes v. Cobb County, 264 Ga. 128, 130 (441 SE2d 406) (1994).
Judgment affirmed.
Birdsong, P. J., Pope, P. J., Johnson and Ruffin, JJ., concur. Beasley, C. J., concurs specially. McMurray, P. J., concurs in judgment only. Andrews, J., dissents. Smith, J., not participating.Due to weather and economic problems, the construction company’s income fell dramatically, experiencing a 90 percent decrease in its gross sales. Creditors repossessed much of the company’s equipment, its insurance policies were cancelled due to lack of payment and the company was forced to fire 25 to 30 of its employees. Moreover, Reynolds had personally guaranteed many of the company’s debts and was being pursued individually by numerous creditors. When construction jobs were available, Reynolds would work seven days a week, often spending over fifteen hours a day at the job site.
Previous cases suggest that stress may be an aggravating factor in cardiovascular problems. See Zippy Mart v. Fender, 170 Ga. App. 617 (317 SE2d 575) (1984).