An inherited disorder of the fibrinolytic system has been discovered as a cause of unusual clotting. An abnormal immunoreactive plasminogen was identified in eight patients who presented with unexplained thrombosis. Six patients presented with spontaneous arterial or venous thrombosis, and two patients developed postoperative occlusion of an arterial reconstruction. Five of the six patients with spontaneous thrombosis had recurrent episodes involving both the arterial and venous system at time intervals between the thrombotic episodes varying from 1 month to several years. Detection of an abnormal plasminogen was made by immunoelectrophoresis of the patient's serum with an antiplasminogen sera. In normal patients, plasminogen migrates as a single band toward the anode. In these eight patients a separate immunoreactive band located nearer the anode and distinct from the normal band was detected. Examination of family members of two patients identified a similar abnormal plasminogen with an overall incidence suggestive of an autosomal dominant inheritance pattern. This study suggests the presence of a genetically determined plasminogen variant resulting in a functional deficiency of the plasminogen system causing a reduction of fibrinolytic activity and a latent thrombotic tendency. Recommended treatment is long-term warfarin anticoagulation.