In this study, 796 men with unstable coronary artery disease (that is, unstable angina or non-Q wave myocardial infarction) were randomized to double-blind placebo-controlled treatment with aspirin (75 mg/day). The long-term efficacy was judged from the occurrence of myocardial infarction or death or severe angina necessitating referral to coronary angiography. The risk of myocardial infarction or death was reduced during aspirin treatment--after 1 year, the risk ratio was 0.52 (confidence interval 0.37 to 0.72). Severe angina necessitating referral to coronary angiography was less common during aspirin therapy--after 3 months, the risk ratio was 0.59 (0.42 to 0.84) and after 1 year 0.71 (0.56 to 0.91). The combined event rate of myocardial infarction or death or referral to coronary angiography was reduced; after 3 months, the risk ratio was 0.44 (0.30 to 0.66) and after 1 year 0.65 (0.54 to 0.79). The 75-mg aspirin dose was well tolerated and had a high level of patient compliance. Treatment with aspirin (75 mg/day) should be recommended to all men for greater than or equal to 3 months after an episode of unstable coronary artery disease. Long-term therapy should be considered if there are no contraindications or side effects.