We performed a randomized, double-blind, placebo-controlled trial in 555 patients with unstable angina who were hospitalized in coronary care units. Patients received one of four possible treatment regimens: aspirin (325 mg four times daily), sulfinpyrazone (200 mg four times daily), both, or neither. They were entered into the trial within eight days of hospitalization and were treated and followed for up to two years (mean, 18 months). The incidence of cardiac death and nonfatal myocardial infarction, considered together, was 8.6 per cent in the groups given aspirin and 17.0 per cent in the other groups, representing a risk reduction with aspirin of 51 per cent (P = 0.008). The corresponding figures for either cardiac death alone or death from any cause were 3.0 per cent in the groups given aspirin and 11.7 per cent in the other groups, representing a risk reduction of 71 per cent (P = 0.004). Analysis by intention to treat yielded smaller risk reductions with aspirin of 30 per cent (P = 0.072), 56 per cent (P = 0.009), and 43 per cent (P = 0.035) for the outcomes of cardiac death or nonfatal acute myocardial infarction, cardiac death alone, and all deaths, respectively. There was no observed benefit of sulfinpyrazone for any outcome event, and there was no evidence of an interaction between sulfinpyrazone and aspirin. Considered together with the results of a previous clinical trial, these findings provide strong evidence for a beneficial effect of aspirin in patients with unstable angina.