Four hemodialysis patients (1 male and 3 females, aged 29-40 years) with unusual recurrent vascular access or dialyzer thrombosis were studied to find out whether a hypercoagulable state exists. Measurements of euglobulin clot lysis time (ELT), fibrinogen, antithrombin III (AT III), protein C (PC), protein S (PS), tissue plasminogen activator (tPA), and plasminogen activator inhibitor (PAI) were done. Results indicated that all patients had prolonged ELT, low tPA, elevated PAI, normal AT III, and PS. Three patients had elevated fibrinogen level and two had low PC. Danazol 200 mg orally once a day effectively prevented any further thrombosis. In 4 weeks’ time, all the abnormal coagulation studies normalized in addition to elevation of AT III, PC and PS. Only 1 female patient had a prolonged menstrual period, which was reversed by lowering the daily dose of danazol to 100 mg. No other side effects were encountered. These data indicate that hypofibrinolysis may play a major role in vascular access or dialyzer thrombosis and that low-dose danazol may provide an effective prophylaxis and treatment. Larger controlled studies are needed to confirm these finding