Abstract Nine patients with antibiotic-associated pseudomembranous colitis were treated with oral vancomycin. All had severe diarrhœa, tissue-culture evidence of a clostridial toxin in stool, and typical lesions on sigmoidoscopic examination, despite discontinuation of all antimicrobials for periods of 10 days to 8 weeks. Oral vancomycin was given in doses of 2 g daily. All patients showed a good clinical response with gradual resolution of diarrhœa over 7 days and a rapid decrease in concentrations of the toxin in stools. Follow-up sigmoidoscopies in seven patients showed major improvement or complete clearing of lesions after 7-10 days of vancomycin treatment. The mean concentration of vancomycin in twenty-five stools obtained during treatment was 3100 μg/g, levels in serum being very low. These results suggest a role for oral vancomycin treatment of antibiotic-associated pseudomembranous colitis which persists for extended periods despite discontinuation of the incriminated antimicrobial.