BACKGROUND: Decreased kidney function has been established as an important risk factor in patients presenting with acute coronary syndrome. In acute coronary syndrome, increased platelet aggregation is associated with vascular complications. The aim of this study is to examine whether decreased kidney function is associated with altered platelet function in patients presenting with acute myocardial infarction. STUDY DESIGN: Prospective cohort. SETTING & PARTICIPANTS: 413 patients presenting with acute myocardial infarction admitted to the cardiac intensive care unit at Ostersund Hospital, Ostersund, Sweden. PREDICTORS: Glomerular filtration rate less than 60 mL/min/1.73 m(2) estimated from serum cystatin C level, comorbidity, medications, and markers of inflammation and hemostasis. OUTCOMES & MEASUREMENTS: Platelet aggregation was assessed by measuring the formation of small platelet aggregates (SPAs) by using a laser light scattering method. A greater SPA level indicates greater platelet aggregation. Platelet aggregation analysis was performed on days 1, 2, 3, and 5 in-hospital. RESULTS: We observed a significant increase in platelet aggregation during the first 3 days in the hospital regardless of kidney function (P