Perforation of colonic diverticula is a complication of corticosteroid use that has not been described in the neurosurgical literature. Between 1987 and 1992, 719 patients who underwent surgery for primary and metastatic brain and spinal tumors of the central nervous system received 2246 to 4936 mg of methylprednisolone given over at least 7 days. Five patients in this group (all men, ages 50-69 yr) experienced a sigmoid diverticular perforation at a mean dose of 3947 mg of methylprednisolone (range, 2240-6160 mg). Of these five, two had a known history of diverticular disease. In contrast, during this same period, 3749 patients who underwent neurosurgical procedures for non-neoplastic conditions did not receive corticosteroids and experienced no colonic perforations. All five patients with colonic perforations presented with abdominal pain and had free intraperitoneal air that was revealed on radiographs of the abdomen. Perforation of a sigmoid diverticulum was confirmed in all five at exploratory laparotomy. Four patients had good outcomes, and one died. We conclude the following: 1) patients over age 50 who receive high-dose corticosteroids are at risk for sigmoid colonic perforation, and these medications should be used with caution in such patients; 2) if possible, lower total doses of perioperative corticosteroids should be used in patients with known diverticular disease; and 3) because corticosteroids mask many of the inflammatory signs of perforation, this diagnosis should be considered in any patient with abdominal discomfort, fever of unknown origin, or unexplained leukocytosis.