A 60-year-old man with dermatomyositis was admitted to our hospital because of dyspnea and hypertension. He had high fever and convulsive seizures after admission. Laboratory examinations showed hemolytic anemia, thrombocytopenia, and renal failure. A clinical diagnosis of thrombotic thrombocytopenic purpura (TTP) was made. He failed to respond to plasma exchange therapy, pulse therapy with methylprednisolone, high-dose gamma-globulin therapy, and antiplatelet therapies with ticlopidine, dipyridamole and a prostacyclin analog of beraprost sodium. He died on his 17th day in hospital. Autopsy examination revealed widespread microthrombi in his kidneys, lungs, spleen, and intestine. Only seven cases of dermatomyositis or polymyositis complicated by TTP have been cited in the literature. TTP was fatal in 6 of these 7 cases. Early diagnosis and prompt treatment may improve the outcome of TTP patients with dermatomyositis. Dermatologists should keep in mind that TTP occasionally arises as a serious complication of dermatomyositis.