The risk of thromboembolic complications in patients with the nephrotic syndrome (NS) is higher than in any other condition encountered in internal medicine. Such thromboembolic complications comprise venous thromboses (calf, thigh, renal vein) with or without pulmonary embolism and arterial thromboses (coronary thromboses, cerebral artery thromboses, peripheral arterial thromboses). Several defects of the plasmatic coagulation system, fibrinolysis and platelet function had been recognized in the nephrotic syndrome. Increased hepatic synthesis causes a rise of coagulation factors, I, II, VII, VIII, X and increased renal loss causes lowering of the plasma concentration of antithrombin III concentration. There is little evidence for DIC. Plasminogen concentration is diminished, whereas total antiplasmin activity is increased. Low alpha-1-antitrypsin concentration secondary to renal loss is outweighed by increased concentrations of other inhibitors especially alpha-2-macroglobulin and alpha-2-antiplasmin. The common presence of material in the urine reacting as fibrin degradation products with passive hemagglutination techniques appears to be proteolytically degraded fibrinogen excreted as a result of non-selective glomerular proteinuria. Platelet counts are normal or slightly elevated and platelet survival time is slightly, decreased. Definite abnormalities of spontaneous aggregation and ADP- or collagen-induced aggregation are demonstrable. Furthermore, arachidonic acid induced platelet aggregation and malondialdehyde formation by platelets of NS patients are increased. Addition of albumin to platelets of NS patients normalises platelet aggregation. This finding points to some acquired defect of platelet function.