Colitis due to Clostridium difficile is diagnosed in 10%-15% of hospitalized patients who develop diarrhea after treatment with antimicrobial drugs. Diagnosis is based on the concurrence of diarrhea, one or more signs or symptoms of enteric intoxication, and stool from which toxigenic C. difficile is isolated or from which its toxins are identified. Clinical trials evaluating therapy may be placebo controlled (for mild disease) or concurrently controlled with an active drug. A randomized, double-blind study design is preferred. Outcome should be assessed by monitoring of the degree of inflammation of the bowel mucosa, the intensity and severity of diarrhea, the duration of illness, changes in stool form, and the eradication of C. difficile or its toxins from stool. Because C. difficile can be a component of the normal gastrointestinal flora, assessment of clinical outcome is paramount.