In six infants and children, acute renal failure, following diarrhea, was associated with hemolytic anemia and thrombocytopenia. This combination of findings, referred to as the hemolytic uremic syndrome, has been recognized with increasing frequency. Four of six patients died despite treatment by transfusion and dialysis. The pathological findings were not uniform and included glomerulonephritis, thrombotic thrombocytopenic purpura, and renal cortical necrosis. The etiology remains obscure. Available evidence suggests that a hypersensitivity process is involved in the pathogenesis.