All 701 heart infarction patients admitted to 15 hospitals in the district of Cottbus between 1981 and 1983 were randomly administered 30, 60 or 1000 mg aspirin daily according to the territorial affiliation of their local hospitals. The physical and drug therapy during the 2 years follow-up was highly standardized; deviations--as far as they occurred--were documented. Lower all-cause mortality was statistically demonstrated in patients over 60 and a lower fatal reinfarction rate in patients over 50 as well as in men. Deaths and fatal reinfarctions were significantly lower among patients with a history of angina pectoris, marked ST-depression, with an infarction location except for the posterior wall and among hypercholesterolemic patients. The preventive effect of 60 mg aspirin daily was less than that of 30 mg in comparison to the 1000 mg group. Side effects were seen in 4 and 8% (first and second year), respectively, of the patients administered 30 mg aspirin as opposed to 22 and 17% in patients allocated 1000 mg. We conclude that the optimum dose of aspirin for preventing reinfarctions could be as low as 30 mg daily.