The presentation of pseudomembranous colitis ranges from mild self-limiting diarrhea to fulminant colitis with overwhelming sepsis. The management of the severe forms of this disease, including the role of surgical intervention, is poorly defined. To evaluate the management and outcome in severe cases, the authors reviewed the records of six patients (four women, two men) seen at The Toronto Hospital between 1985 and 1989 with pseudomembranous colitis manifesting as fulminant colitis. The patients ranged in age from 19 to 69 years (mean 52 years). All presented with nonbloody diarrhea, had peritoneal signs and were severely dehydrated, and all had received antibiotics between 4 days and 6 weeks before the onset of symptoms. The mean preoperative leukocyte count was 40.9 x 10(9)/L. Radiologically, the colon appeared to be dilated in three patients. Two patients were operated on immediately. The other four were treated medically, but three of them required surgery within 24 hours of presentation. Four (67%) of the six patients died. All four had been treated surgically. The mean age of the survivors was 28 years compared with 64 years for those who died. Pseudomembranous colitis can present as severe acute colitis and can carry a high mortality, especially in the aged. Surgical treatment may be required in those who fail to respond to medical management or have peritoneal signs.