Aggregation of platelets is the pathophysiologic basis of the acute coronary syndromes. Eptifibatide, a synthetic cyclic heptapeptide, is a selective high-affinity inhibitor of the platelet glycoprotein IIb/IIIa receptor, which is involved in platelet aggregation. We tested the hypothesis that inhibition of platelet aggregation with eptifibatide would have an incremental benefit beyond that of heparin and aspirin in reducing the frequency of adverse outcomes in patients with acute coronary syndromes who did not have persistent ST-segment elevation. Patients who had presented with ischemic chest pain within the previous 24 hours and who had either electrocardiographic changes indicative of ischemia (but not persistent ST-segment elevation) or high serum concentrations of creatine kinase MB isoenzymes were enrolled in the study. They were randomly assigned, in a double-blind manner, to receive a bolus and infusion of either eptifibatide or placebo, in addition to standard therapy, for up to 72 hours (or up to 96 hours, if coronary intervention was performed near the end of the 72-hour period). The primary end point was a composite of death and nonfatal myocardial infarction occurring up to 30 days after the index event. A total of 10,948 patients were enrolled between November 1995 and January 1997. As compared with the placebo group, the eptifibatide group had a 1.5 percent absolute reduction in the incidence of the primary end point (14.2 percent, vs. 15.7 percent in the placebo group; P=0.04). The benefit was apparent by 96 hours and persisted through 30 days. The effect was consistent in most major subgroups except for women (odds ratios for death or nonfatal myocardial infarction, 0.8 [95 percent confidence interval, 0.7 to 0.9] in men, and 1.1 [0.9 to 1.31 in women). Bleeding was more common in the eptifibatide group, although there was no increase in the incidence of hemorrhagic stroke. Inhibition of platelet aggregation with eptifibatide reduced the incidence of the composite end point of death or nonfatal myocardial infarction in patients with acute coronary syndromes who did not have persistent ST-segment elevation.