BACKGROUND: The aim of the study was to investigate the incidence and clinical consequences of acquired thrombocytopenia in patients with acute coronary syndromes (ACS) in the ACUITY trial. METHODS: We examined 10,836 patients with ACS randomized to receive heparin plus glycoprotein (GP) IIb/IIIa inhibitor, bivalirudin plus GP IIb/IIIa inhibitor, or bivalirudin monotherapy. RESULTS: Acquired thrombocytopenia developed in 740 (6.8%) patients; mild (100,000-150,000 platelets/mm³), moderate (50,000-100,000 platelets/mm³), and severe (< 50,000 platelets/mm³) developed in 656 (6%), 51 (0.5%), and 33 (0.3%) patients, respectively. Patients with acquired thrombocytopenia, compared with those without, were more likely to develop major bleeding (14% vs 4.3%, P < .0001) at 30 days and had higher rates of mortality (6.5% vs 3.4%, P < .0001) at 1 year. By multivariate analysis, acquired thrombocytopenia was an independent predictor of major bleeding at 30 days (hazard ratio [HR] 1.68, 95% CI 1.04-2.72, P = .03). Moderate and severe acquired thrombocytopenia were predictors of mortality at 1 year (HR 2.89, 95% CI 0.92-9.06, P = .06, and HR 3.41, 95% CI 1.09-10.68, P = .03, respectively). Compared to heparin plus GP IIb/IIIa inhibitor, bivalirudin monotherapy was associated with less declines in platelet count by >25% (7.6% vs 5.6%, P = .0009) and >50% (1.4% vs 0.7%, P = .004) from baseline. CONCLUSIONS: Acquired thrombocytopenia occurs in approximately 1 in 14 patients with ACS treated with antithrombin and antiplatelet medications and is strongly associated with hemorrhagic and ischemic complications. Compared to an anticoagulant regimen including a GP IIb/IIIa inhibitor, administration of bivalirudin monotherapy appears to be associated with less frequent declines in platelet count.