Nine cases of pseudomembranous colitis (PMC) were observed during a 20-month period. All patients had received, or were taking, antibiotics, in five cases lincomycin. The clinical syndrome of PMC occurred in two patients after a major gastrointestinal operation, in two after fracture of the neck of the femur, and in the remaining five, after administration of antibiotics for inflammatory diseases. The clinical syndrome was characterized by an acute onset of profuse diarrhea, pyrexia, abdominal pain, dehydration, and in four patients confusion or hypotension. The diagnosis was made on the basis of rectosigmoidoscopy and histology. No attempt was made to isolate Cl. difficile or to identify neutralizable fecal toxin. All patients received metronidazole at a dose of 1.5 gr. daily with a good response. Eight patients recovered fully. Only one died.