Despite the well-recognized role of platelets in the pathogenesis of acute myocardial infarction (AMI) and in the vascular responses to angioplasty, the relation between platelet count and outcomes after primary percutaneous coronary intervention (PCI) in AMI is unknown. We therefore determined the effect of baseline platelet count on clinical and angiographic outcomes of patients with AMI undergoing primary PCI. In the prospective, randomized CADILLAC trial, platelet count on admission was available in 2,021 of 2,082 patients (97.0%). Angiographic results and outcomes at 30 days and 1 year were stratified by quartiles of platelet count. Median platelet count was 231 x 10(9)/L (range 38 to 709). Primary PCI angiographic success rates were independent of platelet count. The 30-day incidence of target vessel thrombosis or reocclusion increased steadily across the higher quartiles of baseline platelet count (0.2%, 0.6%, 1.0%, and 2.0%, p = 0.027). At 1 year, patients with a baseline platelet count >or=234 versus <234 x 10(9)/L had higher rates of death or reinfarction (8.9% vs 4.5%, p <0.0001), death (5.8% vs 3.1%, p = 0.002), and reinfarction (3.4% vs 1.6%, p = 0.008). By multivariable analysis, a higher baseline platelet count was the strongest predictor of 1-year death or reinfarction (hazard ratio [HR] per 10,000 increase in platelet count 1.02, 95% confidence interval [CI] 1.02 to 1.07, p <0.0001) and independently predicted reinfarction (HR 1.06, 95% CI 1.02 to 1.09, p = 0.002) and cardiac mortality (HR 1.03, 95% CI 1.00 to 1.06, p = 0.055) at 1 year. In conclusion, a higher baseline platelet count in patients with AMI is a powerful independent predictor of death and reinfarction within the first year after primary PCI.