It has been previously demonstrated that aging, a history of malignancy or surgery, and exposure to bovine thrombin may be related to the presence of factor V inhibitor. However, dermatomyositis (DM) and autoimmune thrombocytopenic purpura (ATP) have rarely been associated with factor V inhibitor. Here we report a patient with factor V inhibitor accompanied by DM and ATP. In January 2000, a 77-year-old woman with DM was hospitalized because of susceptibility to bleeding. At the onset of DM, she had suffered from gastric leiomyosarcoma, and had undergone gastrectomy and splenectomy without the use of bovine thrombin. Thereafter, she had been treated with prednisolone until October 1999. On admission, prolongation of both APTT and PT was seen. Her factor V activity had fallen to 6%, and factor V inhibitor was positive at 8.9 Bethesda units. Moreover her platelet count had dropped to 1.0 x 10(9)/l. A bone marrow aspirate showed a cellular marrow with megakaryocytic hyperplasia, and the patient's PA-IgG level was elevated at 389 ng/10(7) cells. These findings suggested that ATP was also present. Additionally, the patient showed a positive result for Coombs test and consumption of complement. After a further course of steroid therapy, the patient's condition was markedly improved. This is a very rare case that showed factor V inhibitor and ATP simultaneously. Furthermore, the patient's clinical course suggests the relationship between the presence of factor V inhibitor and the reactivation of her collagen disease activity.