A patient with acquired factor X deficiency due to amyloidosis had a prolonged thrombin time. Investigation of this abnormality suggested that an inhibitor of thrombin was present. A review of the literature demonstrated that prolonged thrombin time or reptilase time was present in many of the patients with acquired factor X deficiency due to amyloidosis. We were unable to relate prolongation of the thrombin time or reptilase time to the severity of bleeding. We believe that determination of thrombin and reptilase times should be part of the coagulation profile in all patients with acquired factor X deficiency.