Thrombotic thrombocytopenic purpura (TTP) is characterized by thrombocytopenia and microangiopathic hemolysis. Unlike the typical hemolytic uremic syndrome (HUS), which follows infection with shiga toxin-producing microorganisms, most cases of TTP do not have an obvious etiology. Recent studies revealed that a plasma zinc metalloprotease ADAMTS (a disintegrin and metalloprotease with thrombospondin type 1 motif) 13 cleaves von Willebrand factor in a shear-dependent manner. Deficiency of ADAMTS13, due to autoimmune inhibitors of the protease or genetic mutation in the ADAMTS13 gene, results in a propensity to the development of von Willebrand factor-platelet aggregation and intravascular thrombosis characteristic of TTP. The identification of the molecular defect in TTP raises the prospect that this hitherto mysterious disorder will be managed with a more rationally designed strategy in the near future.