Verocytotoxin-producing Escherichia coli (VTEC) infection was present in three cases of hemolytic uremic syndrome (HUS), two fatal and one non-fatal, in which detailed histopathologic investigations were conducted. Two patients had a prodrome of bloody diarrhea, one of whom required a hemicolectomy for severe bleeding. The renal histopathology was characterized primarily by glomerular thrombotic microangiopathy (TMA) with greater than 95% of glomeruli showing changes of capillary wall thickening, endothelial cell swelling, and narrowing or thrombosis of the capillary lumen. Preglomerular arterioles were frequently thrombosed, and abnormalities of the medium-sized vessels, including endothelial cell damage and thrombosis, were also commonly observed. Gastrointestinal involvement was prominent in all three cases. The colon was most severely involved, with marked mucosal and submucosal edema and hemorrhage, in the absence of significant inflammation or widespread ulceration. Microvascular angiopathy was present in all cases, with changes ranging from endothelial cell damage to overt thrombosis. Similar pathology was seen throughout the small bowel, including the presence of TMA. In one patient, typical morphologic changes of pseudomembranous enterocolitis were found in the absence of infection with Clostridium difficile. The nature of vascular involvement in the kidneys and intestinal tract supports the hypothesis that HUS is mediated by systemic toxemia, and that endothelial cells are the primary target cells for the action of verocytotoxin.