BACKGROUND: The significance of thrombocytopenia in patients experiencing an acute coronary syndrome (ACS) has not been examined systematically. We evaluated this condition in a large non-ST-elevation ACS clinical trial, with particular interest paid to its correlation with clinical outcomes. METHODS AND RESULTS: Patients presenting without persistent ST elevation during an ACS were randomized to receive a double-blind infusion of the platelet glycoprotein (GP) IIb/IIIa inhibitor eptifibatide or placebo in addition to other standard therapies including heparin and aspirin. The primary end point was death/nonfatal myocardial infarction (MI) at 30 days, whereas bleeding and stroke were the main safety outcomes. Thrombocytopenia (nadir platelet count <100x10(9)/L or <50% of baseline) occurred in 7.0% of enrolled patients. The time to onset was a median of 4 days in both treatment arms. Patients with thrombocytopenia were older, weighed less, were more likely nonwhite, and had more cardiac risk factors. These patients experienced significantly more bleeding events: they were more than twice as likely to experience moderate/severe bleeding after adjustment for confounders. Univariably, ischemic events (stroke, MI, and death) occurred significantly (P<0.001) more frequently in patients with thrombocytopenia; multivariable regression modeling preserved this association with death/nonfatal MI at 30 days. Neither the use of heparin or eptifibatide was found to independently increase thrombocytopenic risk. CONCLUSIONS: Although causality between thrombocytopenia and adverse clinical events could not be established definitively, thrombocytopenia was highly correlated with both bleeding and ischemic events, and the presence of this condition identified a more-at-risk patient population.