BACKGROUND: Stroke is a rare but potentially devastating complication of acute myocardial infarction. Little is known about stroke timing, characteristics, and clinical outcomes in patients with ST-segment elevation myocardial infarction treated with primary percutaneous coronary intervention (PCI). METHODS AND RESULTS: We studied 5372 patients enrolled in the Assessment of Pexelizumab in Acute Myocardial Infarction (APEX-AMI) trial. We analyzed stroke incidence, type, timing, and association with the prespecified 90-day clinical outcomes. Cox proportional hazards modeling was performed to assess the relationship between stroke and outcomes, after adjusting baseline characteristics and analyzing stroke as a time-dependent covariate. Stroke occurred in 69 primary patients with PCI (1.3%). A third of strokes were ischemic (n=23; 33%), 12% (n=8) were hemorrhagic, and the remaining 55% (n=38) were of uncertain type. The median (25th, 75th percentile) time of stroke occurrence was 6 (3, 14) days. Overall, 43% of strokes occurred within 48 hours of PCI, and all hemorrhagic strokes occurred within 48 hours. Stroke was associated with an increased risk of 90-day death (unadjusted hazard ratio [HR], 8.0; 95% confidence interval [CI], 4.8-13.5), congestive heart failure (unadjusted HR, 3.2; 95% CI, 1.3-7.8), and 30-day hospital readmission (unadjusted HR, 3.2; 95% CI, 2.0-5.1). After adjustment, stroke was still strongly associated with 90-day death (adjusted HR, 5.6; 95% CI, 3.2-9.8) and the combination end point of death, congestive heart failure, or cardiogenic shock at 90 days (adjusted HR, 2.4; 95% CI, 1.2-4.7). CONCLUSIONS: Stroke is an infrequent complication in the setting of ST-segment elevation myocardial infarction treated with primary PCI but is associated with increased morbidity and mortality. Studies to determine mechanisms that may be responsible for strokes that occur >48 hours from primary PCI are warranted. CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrials.gov. Unique identifier: NCT00091637.