A case of nosocomial diarrhea by Clostridium difficile in an older woman with an old history of increasing stool frequency, is reported. Colonoscopy and biopsy was performed due to an incomplete response to vancomicyn, and the diagnosis of underlying ulcerative colitis was made. The incidence of Clostridium difficile infection associated with the relapse of ulcerative colitis is nearly 10%. In patients with ulcerative colitis, macroscopic pseudomembranes and the usual predisposing factors for Clostridium difficile infection, usually, are not present. It seems to exist a significant correlation between the severity of the relapse and Clostridium difficile. The specific treatment of the pseudomembranous colitis, in the majority of the cases, is sufficient for a correct control of relapse of ulcerative colitis.