BACKGROUND: Clostridium difficile is a frequent cause of serious nosocomial infection. Earlier reports have suggested that treatment with metronidazole cured nearly 90% of patients, with only a modest rate of recurrence of infection. In recent years, the rate of response to treatment with this drug has appeared to be much lower. METHODS: We undertook a prospective, observational study of 207 patients who were treated with metronidazole for C. difficile colitis. RESULTS: A total of 103 patients (50%) were cured by the initial course of therapy and had no recurrence of disease. Forty-six patients (22%) continued to have symptoms of colitis for > or = 10 days despite treatment, and 58 (28%) responded initially but had a recurrence within the ensuing 90 days. The mortality rate among patients who developed C. difficile colitis was 27%, and it was higher among patients who did not respond fully to an initial course of therapy, compared with those who did (33% vs. 21%; P < .05). CONCLUSIONS: Because of the relatively poor response to therapy, additional approaches to prevention and/or treatment of C. difficile colitis appear to be warranted.