Platelet prothrombinase activity and microvesicle (MV) generation were measured in four patients from three unrelated families with a life-long bleeding disorder associated with slightly prolonged bleeding time and isolated defective serum prothrombin consumption, without platelet function abnormality or von Willebrand factor defect. MV generation was reduced in all the patients either after thrombin plus collagen or A23187 calcium ionophore stimulation, whereas, at variance with Scott syndrome, prothrombinase activity was normal. This abnormality constitutes a new bleeding disorder, which provides new insights into the possible role of platelet microvesicles in health and disease. Furthermore, the results of this study suggest that MV generation should be investigated in patients with a bleeding history and apparently isolated prolonged bleeding time when prothrombin consumption in serum is defective and all other investigations are normal.