Thrombotic thrombocytopenic purpura (TTP) and hemolytic-uremic syndrome (HUS) are acute, fulminant disorders characterized by hemolytic anemia with erythrocyte fragmentation, thrombocytopenia, and renal impairment. Although TTP and HUS are often indistinguishable, TTP is often accompanied by neurologic abnormalities (commonly with mental status changes), while HUS typically involves renal dysfunction (see Table 36.1). Thrombotic microangiopathy (TMA) is an inclusive term that describes the morphologic changes of the microvasculature seen in both TTP and HUS. However, TMA cannot be used synonymously for TTP and HUS because TMA is also caused by pregnancy and malignant hypertension. TMA caused by TTP and HUS is described in this section.