Prolonged bleeding time and alteration in clotting factors were observed in a patient with polycystic renal disease and cerebral artery aneurysm who received pharmacological doses of dextran-40. It is suggested that these abnormalities may have been aggravated by selective retention of the higher molecular weight fraction of the polymer, due to increased renal decompensation induced by the dextran. The dextran effect on platelet function is probably the major cause of impaired hemostasis. Other coagulation changes such as reduced Factor VIII and fibrinogen levels, shortened thrombin and reptilase times, and the possible mechanisms of action involved are discussed.