Abstract A study was made of nine fatal and three non-fatal cases of pseudomembranous enterocolitis occurring postoperatively during a recent five-year period. Only two of these patients underwent operative procedures on the colon. Fever and diarrhea were the most common initial manifestations of the disease, and appeared an average of 5.7 days after operation. Prior to the onset of enterocolitis all patients had received antimicrobial therapy, which in almost all instances was given by the parenteral route only. Gram-positive cocci were identified as the sole or predominant organism in stool smears, and hemolytic Staph. aureus was cultured from the stools of all but one patient on whom these studies were performed. The importance of prompt diagnosis and treatment is indicated by an average delay of only 5.7 hours in initiating therapy in those patients who survived, and a delay of forty hours in fatal cases. Death occurred an average of less than five days after the onset of symptoms. Pathologic changes were typical of those previously described and usually involved extensive areas of the colon, small intestine and, occasionally, the stomach. Evidence from this and other recent studies indicates that alteration of the bowel flora by broad-spectrum antibiotics with resultant overgrowth of the hemolytic Staph. aureus is responsible for most cases of pseudomembranous enterocolitis at the present time. Three objectives should be sought in treatment: elimination of hemolytic staphylococci and establishment of the normal flora of the bowel; restoration of water, electrolytes and protein in the body; and reduction of toxicity.