Abstract A 79 year old white woman presented with a severe bleeding disorder. Evaluation revealed a prothrombin time of 27.6 seconds (control, 11 seconds) and an activated partial thromboplastin time of 61 seconds. Specific clotting factor assays showed an isolated deficiency of factor X ranging from 7 to 12 per cent on three determinations. Platelet aggregation and bleeding time were also abnormal in response to epinephrine and collagen. Factor X levels and platelet aggregation returned to normal and bleeding stopped after institution of corticosteroid therapy.