The pathogenesis of TTP appears to result from a deficiency of ADAMTS-13, the protease responsible for the proteolytic cleavage of large hemostatically hyperactive vWf multimers, in plasma into smaller less adhesive multimers. ADAMTS-13 prevents inappropriate microvascular platelet aggregation. Deficiency of ADAMTS-13 is a finding for TTP, a disorder that manifests as thrombocytopenia, microangiopathic hemolytic anemia, renal failure, neurologic dysfunction, and fever, the classic pentad of clinical features. Platelet-rich microvascular thrombi are responsible for the renal and cerebral infarcts as well as damage to other organ systems. In the absence of ADAMTS-13, large, hyperactive, multimers of vWf circulate in the plasma. The large hyperactive multimers are secreted from the endothelial cells and cause disseminated platelet clumping in the microvasculature.