We collected 23 autopsied cases of thrombotic thrombocytopenic purpura (TTP) and examined them immunohistochemically and electronmicroscopically to elucidate the nature of thrombi and subendothelial deposits. The findings were compared with those of 10 cases of disseminated intravascular coagulation (DIC) and 3 cases of polyarteritis nodosa (PN). Intravascular thrombi and subendothelial hyaline deposits in TTP stained weakly for fibrinogen/fibrin by the PAP technique, while they stained strongly for factor VIII related antigen (FVIIIR: Ag). Electronmicroscopically, thrombi in TTP were composed of numerous, variably degranulated and altered platelets, and a small amount of amorphous materials. Thrombi in DIC were strongly positive for fibrinogen/fibrin, while they were weakly positive for FVIIIR:Ag. Electronmicroscopically they were composed of polymerized fibrin. Fibrinoid necrotic lesions in PN were strongly positive for fibrinogen/fibrin but negative for FVIII R:Ag. Based on these findings, we concluded that thrombi in TTP are essentially platelet thrombi and that subendothelial hyaline deposits may not be a result of increased vascular permeability but may be platelet thrombi incorporated into the arterial wall and covered with newly formed endothelial cells.