To study possible triggering of first events of acute myocardial infarction by heavy physical exertion, the authors conducted a case-crossover analysis (1993-1994) within a population-based case-referent study in Stockholm County, Sweden (the Stockholm Heart Epidemiology Program). Interviews were carried out with 699 myocardial infarction patients after onset of the disease. These cases represented 47 percent of all cases in the study base, and 70 percent of all nonfatal cases. The relative risk from vigorous exertion was 6.1 (95% confidence interval: 4.2, 9.0). The rate difference was 1.5 per million person-hours, and the attributable proportion was 5.7 percent. The risk was modified by physical fitness, with an increased risk being seen among sedentary subjects as in earlier studies, but the data also suggested a U-shaped association. In addition, the trigger effect was modified by socioeconomic status. Premonitory symptoms were common, and this implies risks of reverse causation bias and misclassification of case exposure information that require methodological consideration. Different techniques (the use of the usual-frequency type of control information, a pair-matched analysis, and a standard case-referent analysis) were applied to overcome the threat of misclassification of control exposure information. A case-crossover analysis in a random sample of healthy subjects resulted in a relative risk close to unity, as expected.