Platelet aggregation, malondialdehyde (MDA) production, and recovery after aspirin (ASA) administration and plasma levels of beta-thromboglobulin (BTG) were determined in 40 asymptomatic patients with mitral valve prolapse (MVP) and 17 normal subjects. Platelet aggregation was similar in patients and controls, although two patients presented spontaneous aggregation. Production of MDA and plasma levels of BTG were higher in MVP than in controls; however, recovery after ASA was similar in both groups. The results further indicate that platelet hyperactivity is present in a significant number of asymptomatic patients with MVP.