['Thrombotic thrombocytopenic purpura (TTP) and the hemolytic uremic syndrome (HUS) share many clinical features and have been difficult to separate into distinct entities. Histologic examination of organs from autopsied patients suggested that TTP and HUS have dissimilar lesions of different severity and distribution.', 'To perform a retrospective observational review of autopsied patients with TTP or HUS to compare the nature and severity of the lesions found.', 'To examine the pathologic features of these conditions, we reviewed all cases among 51 350 indexed autopsies at The Johns Hopkins Hospital (Baltimore, Md) diagnosed with TTP or HUS, and included those showing multiple arteriolar thrombi or their sequela.', 'The 56 cases that met the inclusion criteria fell into 2 distinct groups, based on distribution and severity of arteriolar lesions. In 25 patients classified as having TTP, platelet-rich thrombi were present-in decreasing severity-in heart, pancreas, kidney, adrenal gland, and brain. In 31 patients with HUS, fibrin/red cell-rich thrombi were present, largely confined to the kidney and often severe, and only 6 cases showed pancreas involvement, 4 adrenal gland involvement, 2 brain involvement, and 1 heart involvement.'] Are thrombotic thrombocytopenic purpura and hemolytic uremic syndrome distinct pathologic entities . A review of 56 autopsy cases?