Pseudomembranous colitis is caused by release of toxins from Clostridium difficile when it colonizes the large intestine. This clostridium is susceptible to concentrations of vancomycin which are readily attained in the colon after oral administration. When vancomycin is given orally to infected patients in a dose of 125 mg every 6 h, a rapid clinical cure can be expected. Some patients may relapse after the vancomycin is stopped, but a further course of treatment will control symptoms.