Background Sudden death (SD) and acute myocardial infarction (AMI) are the main complications limiting long-term survival after heart transplantation (HT). They are unpredictable and, at present, unpreventable. Platelet aggregation (PA) has recently emerged as a significant prognostic indicator in nontransplanted coronary disease patients. The main purpose of the present study was to evaluate to what extent PA could predict SD and AMI in long-term survivors of HT independently of serum lipid levels.36%) or low (<36%) ADP-induced platelet aggregation was 4.3 (95% confidence interval, 1.9 to 9.5, P=.0001).Conclusions This study provides the first demonstration of an association between increased platelet aggregation and subsequent SD or AMI in HT recipients. It suggests that platelets and thrombosis also are implicated in the pathogenesis of AMI and SD in HT recipients. Identification of a safe and effective antiplatelet therapy should be actively pursued. (Circulation. 1994;89:2590-2594.)