OBJECTIVE Toxic megacolon is a rare complication of pseudomembranous enterocolitis. We reviewed our recent experience with this complication. METHODS The first five patients of the series were studied retrospectively, and six others were followed prospectively. RESULTS Between June 1992 and May 1994, 11 patients (8 male, 3 female) developed toxic megacolon secondary to pseudomembranous enterocolitis. Mean age was 60.7 +/- 11.8 (range, 40-79) years. Presenting symptoms and signs included diarrhea, 100 percent; malaise, 91 percent; abdominal pain, 82 percent; abdominal distention, 82 percent; abdominal tenderness, 72 percent; anemia less than 12 gm, 72 percent; albumin less than 3 gm, 64 percent; tachycardia greater than 100, 55 percent; fever greater than 38.5 degrees celsius, 45 percent; shock or hypotension, 45 percent. Predisposing factors included antibiotics, 64 percent; immunosuppressants or chemotherapy, 36 percent; antidiarrheals, 27 percent; and barium enema in one patient. Five patients (45 percent) had more than one predisposing factor. X-rays showed transverse colon dilation and loss of haustrations in eight patients (72 percent), with a mean diameter of 9.9 +/- 3.4 cm. Flexible proctosigmoidoscopy showed pseudomembranes in all scoped patients, and toxin assay for Clostridium difficile was positive in all patients. One patient had emergency surgery. Ten patients were initially treated medically with nasogastric suction and intravenous resuscitation (90 percent) and antibiotics (100 percent), usually in the intensive care unit (80 percent). Four patients did not respond and underwent surgery; two others improved, then deteriorated, and also underwent surgery. Altogether, 7 of 11 patients (64 percent) underwent surgery. Three patients (27 percent) responded well to medical treatment. One patient was deemed too ill to undergo surgery and died. Mean delay to surgery was 3.0 +/- 1.3 days. No sealed or overt perforation was found at laparotomy. All patients who underwent surgery had a subtotal colectomy, with either a Hartmann's stump (71 percent) or a mucous fistula (29 percent). Eventually, five of seven patients who were operated on and two of four medically treated patients died (overall mortality, 64 percent). Only one patient underwent closure of ileostomy and anastomosis. CONCLUSIONS Toxic megacolon complicating pseudomembranous enterocolitis is a serious problem that carries a high morbidity and mortality rate, regardless of treatment.