Abstract
"Thrombotic thrombocytopenic purpura" is a thrombocytopenic, hemolytic disease with occlusive mounds in the arterioles and capillaries. These mounds have been popularly supposed to represent bland platelet thrombi. In recent years observations have been forthcoming in favor of a primary vascular lesion as the anatomic basis of the disease. This paper places special emphasis on the latter proposition, and pathologic material is presented to demonstrate how such a lesion probably evolves.