Thrombotic thrombocytopenic purpura (TTP) and the hemolytic uremic syndrome (HUS) are both characterized by thrombocytopenia, microangiopathic hemolysis, and organ dysfunction. Other disorders occasionally present with similar manifestations. Recent studies have demonstrated that deficiency in the von Willebrand factor cleaving protease ADAMTS13, due to genetic mutations or autoimmune inhibitors, causes TTP. Molecular cloning of ADAMTS13 elucidates the structure of the protease, raising the prospect for advances in diagnosis and treatment of the disease. Assay of ADAMTS13 activity distinguishes TTP from HUS and other types of thrombotic microangiopathy (TMA); therefore, the term TTP/HUS should be avoided because it obscures the known or potential differences among the various types of TMA.