Spontaneous platelet aggregation (SPA) has been recently reported to occur in patients with occulsive diseases. The clinical implication of SPA is however not clarified yet. To investigate the clinical importance and mechanism of this phenomenon, 287 patients who were referred to us for platelet aggregation study were, studied. SPA was positive in 19 patients, 5 in cerebral thrombosis, 3 in Behçet disease, 2 in angina pectoris and 5 in miscellaneous thrombotic disorders.Platelets from these patients can aggregate with extremely small amount of aggregating agents such as ADP, epinephrine and collagen. Maximum aggregation of SPA depends on platelet counts in platelet rich plasm. On several occasions, SPA became significant only after platelet rich plasma was kept at room temperature for 1-2 hours. SPA was markedly inhibited by apyrase at a concentration of 0.1mg/ml. Aspirin intake at a dose of 500mg daily for one week abolished SPA. Electrophoretic mobilities of platelets from 2 patients with Behçet disease who had SPA were significantly reduced. SPA platelets showed no changes in sialic acid levels or membrane glycoprotein profiles on SDS polyacrylamide gel disc electrophoresis. Malondialdehyde levels of SPA platelets were higher than normal platelets.Our study clearly indicates that SPA was frequently found in thrombotic disorders. SPA platelets were more sensitive to aggregating agents, and release of ADP plays a role in development of SPA. The mechanism of SPA remained unsolved, but decreased electrophoretic mobility and elevated malondialdehyde level of SPA platelets may give some clue for further study.