Pseudomembranous colitis is an increasingly frequent nosocomial infection; it commonly occurs after the use of broad-spectrum antibiotics, that allow overgrowth of Clostridium difficile. Although this microorganism has been associated with relapse of ulcerative colitis, pseudomembranous colitis complicating inflammatory bowel disease is unusual. We report a 63-year-old female with fulminant pseudomembranous colitis complicating ulcerative colitis that required surgery. Colitis due to C. difficile should be considered in patients with ulcerative colitis previously treated with antibiotics.