The clinical syndrome of fever, neurologic abnormalities, renal impairment with laboratory findings of thrombocytopenic and microangiopathic hemolytic anemia is seen in thrombotic thrombocytopenic purpura (TTP) and a variety of disorders associated with thrombotic microangiopathy (TMA). With improved understanding of the pathogenesis of the perturbed metabolic pathway of von Willebrand factor in TTP, the classic Moschcowitz syndrome, now more accurately referred to as idiopathic TTP, can be distinguished from other TMAs. The distinguishing features are useful not only in providing an accurate diagnosis but also help to determine the best therapeutic strategy.