To evaluate whether vasectomy is associated with a subsequent increase in the incidence of myocardial infarction 10 or more years after surgery and whether an effect is more pronounced in those already predisposed to a myocardial infarction, a hospital-based case-control study was carried out in men aged less than 55 years. The men were interviewed during 1980-1983 in 78 hospitals in Massachusetts, Rhode Island, Connecticut, and New York. Among 2,238 men with first episodes of myocardial infarction, 332 (15%) had undergone vasectomy, compared with 572 (16%) of 3,361 controls. Vasectomy greater than or equal to 10 years earlier was reported by 150 cases and 180 controls, to yield a multivariate relative risk estimate of 1.0 after allowance for potential confounding factors (95% confidence interval (Cl) = 0.8-1.3); for vasectomy greater than or equal to 15 years before, based on 34 cases and 33 controls, the estimate was 1.1 (95% Cl = 0.7-2.0). In men predisposed to myocardial infarction because of more advanced age, cigarette smoking, elevated cholesterol level, hypertension, angina pectoris, or other risk factors, vasectomy did not appear to increase the risk further, even after intervals of greater than or equal to 10 years. The results provide evidence against an increased risk of myocardial infarction greater than or equal to 10 years after vasectomy overall and in those known to be predisposed because of other risk factors.