A prospective, randomized study comparing oral teicoplanin with oral vancomycin in the treatment of pseudomembranous colitis (PMC) and Clostridium difficile-associated diarrhea (CDAD) was performed. Teicoplanin was administered at a dosage of 100 mg twice a day for 10 days, and vancomycin was administered at a dosage of 500 mg four times a day for 10 days. CDAD was diagnosed by demonstrating both C. difficile and cytotoxin in the feces of symptomatic patients (more than three loose stools per day). The diagnosis of PMC was also based on colonoscopy. Cytotoxin assay and cultures were checked in all patients 7 to 10 days after discontinuation of therapy and 25 to 30 days thereafter. Of the 51 patients enrolled, 46 were judged to be assessable. Among these, 26 received teicoplanin and 20 received vancomycin. At enrollment, both groups were comparable in terms of age, sex, occurrence of PMC or CDAD, and previous antibiotic treatment. Eighteen of the 20 patients in the vancomycin group and 10 of the 26 patients in the teicoplanin group had previously undergone surgery (P = 0.0004). Treatment resulted in the clinical cure of 20 (100%) vancomycin and 25 (96.2%) teicoplanin patients (P = 0.56). After discontinuation of therapy, clinical symptoms recurred in four (20%) vancomycin patients and two (7.7%) teicoplanin patients (P = 0.21). Posttherapy asymptomatic C. difficile carriage (positive follow-up cultures without any clinical symptoms) occurred in five (25%) vancomycin patients and two (7.7%) teicoplanin patients (P = 0.11).Overall, 9 of 20 (45%) vancomycin patients and 5 of 26 (19.2%) teicoplanin patients (P=0.059) appeared not to be cleared of C. difficile after treatment. No adverse effects related to vancomycin or teicoplanin therapy were observed.