Patients with acute myocardial infarction (AMI) admitted between 1986 and 1989 were divided into aspirin (ASA) group (216 cases) and control group (211 cases). In ASA group, 50 mg ASA was given daily to each case from the early stage of AMI and continuously after discharge (follow-up in OPD). In control group, no antiplatelet agents were administered during the follow-up period. The other therapies were comparable in both groups. The patients in both groups were scheduled for examination in the OPD every one to two months after discharge. Both groups were followed up until July, 1990. The follow-up periods in ASA group and control group were 19.4 +/- 12.6 vs 20.7 +/- 13.0 months respectively. There were 312 men in this study (175 in ASA group and 137 in control group). In men, incidence of reinfarction was reduced by 65% (P less than 0.001) and platelet aggregation was inhibited obviously in ASA group as compared to control group. However, there were no statistically significant differences of reinfarction and platelet aggregation between ASA group and control group in women. Thus, it is evident that low dose ASA is effective in preventing reinfarction and inhibiting platelet aggregation in men.