Acquired factor X deficiency has been described in patients with amyloidosis but acquired factor V deficiency is quite rare. We report here a case of life-threatening bleeding and acquired factor V deficiency associated with primary amyloidosis. A 50-year-old man who had no previous hemorrhagic diathesis was referred to our hospital because of recurrent epistaxis, gingival bleeding and hemospermia. The laboratory examination revealed that both the prothrombin time (PT) and the activated partial thromboplastin time (aPTT) were significantly prolonged, and factor V activities were markedly decreased to 14-39% of the normal value. Other coagulation factors such as fibrinogen, prothrombin, factor VII, factor VIII, factor IX and factor X were subnormal and normal. Transaminases were slightly elevated but serological tests of hepatitis B and hepatitis C were negative. Mild hepatosplenomegaly was noted without sign of liver cirrhosis. The PT and aPTT obtained 8 years ago when he received a cholecystectomy due to cholecystitis were both normal. Specific assays for the detection of factor V inhibitor were repeatedly performed but no factor V inhibitor was found. Furthermore, a significant recovery of the infused factor V was noted shortly after an intravenous administration of 5-10 U fresh frozen plasma, but it did not last more than 6 h. Melena, bleedings into the left shoulder and buttock, and finally mortal retroperitoneal hemorrhage developed despite repeated infusions of large amounts of fresh frozen plasma. Acquired factor V deficiency associated with primary amyloidosis was suspected but histological diagnosis was not obtained because of the severe bleeding tendency. Autopsy revealed hepatosplenomegaly and massive deposits of AL amyloid in the liver, spleen, heart and other parenchymal organs. Perivascular amyloid deposition and factor V deficiency are both thought to be the cause of the severe hemorrhagic tendency seen in this patient.