Necrotizing enterocolitis occurring in leukemia is not well appreciated. In a recent 18-month interval this lesion was found in 12% of leukemia cases in which autopsies were done. A segment of bowel exhibiting necrosis, edema, and hemorrhage were noted in the distal ileum, the cecum, or the ascending colon. Usually the correct diagnosis was not suspected antemortem. Clinical findings were fever, diarrhea, ileus, and abdominal pain. Peritonitis and shock developed later. Treatment, which included surgical intervention in two cases, was unsuccessful. All our patients died. The etiology probably involves many factors including prior treatment with broad-spectrum antibiotics, chemotherapeutic agents, severe granulocytopenia, thrombocytopenia, bacteria, and fungi.