A 62-year-old woman was admitted for evaluation of muscular weakness, skin pigmentation, dry mouth, and interstitial pneumonia. During the course of her stay, adult respiratory distress syndrome, hemolytic anemia, renal failure, neurologic dysfunction, and thrombocytopenia appeared. A clinical diagnosis of thrombotic thrombocytopenic purpura (TTP) accompanied by polymyositis and Sjögren's syndrome was made. She died two weeks after the beginning of plasmapheresis, and an autopsy was performed. Immunohistochemistry disclosed deposits of IgM, fibrinogen, and C1q in glomeruli and arterioles and deposits of C3 in small arteries. von Willebrand's factor antigen, which promotes the adhesion of platelets to the subendothelium, was positive in onion-peeled arteries of the kidney and the spleen. These results suggest that immune complexes may have triggered a sequence of events from vascular endothelial injury to TTP.