The abnormalities of the hemostatic mechanism in three consecutive patients with acute renal insufficiency have been studied. In every case a prolonged clotting time in silicone and a prolonged bleeding time were found. In two cases defective prothrombin consumption was also present. Coagulation studies revealed no significant deficiency of plasma factors, no serious deficiency of platelet numbers or function and no well-defined anticoagulant effect to explain the changes. All three cases developed significant clinical bleeding at some time during the course of the renal insufficiency.