The clinical courses of 114 patients with positive Clostridium difficile cultures or toxin assays performed between 1981 and 1984 were reviewed to determine the relationship between outcome of treatment and quantitative bacteriologic test results. C. difficile culture was positive in 60 of 91 patients while toxin assay was positive in 99 of 114. One third of the patients received supportive therapy only, and 30 percent of these failed to resolve their symptoms. Ninety-one percent of the patients treated with vancomycin resolved, although 11 percent of these suffered relapse. Patients with high toxin titers receiving supportive treatment alone showed a lower response rate than patients with lower toxin titers. This effect was not seen in patients treated with specific therapy nor with different culture quantities. C. difficile colitis has a range of clinical and microbiologic manifestations. Endoscopy is not always diagnostic, both culture and toxin assays are needed for diagnosis, and toxin titer may help in planning treatment. Patients with low toxin titers may be treated supportively, but high toxin titers are an indication for specific therapy. Quantitative culture results have little diagnostic or therapeutic value.