The plain abdominal film findings in nine renal transplant patients with pseudomembranous colitis were characteristic of but not specific for this entity and most commonly showed "thumb-printing" reflecting submucosal edema (five patients) and a persisting localized segmental ileus in the colon (five patients). Based on the plain film findings, the differential diagnosis will include the acute stages of ulcerative colitis, granulomatous colitis, ischemic colitis and other inflammatory colitides, but the radiologist is in a position to suggest the correct diagnosis in the appropriate clinical setting. A barium study of the colon is not indicated, as diagnosis is usually accomplished by endoscopy. The etiology of this colitis has recently been shown to be a cytopathic toxin of the anaerobic bacterium, Clostridium difficile.