Twenty-seven patients with chronic diarrhea due to Crohn's disease, ulcerative colitis, short bowel syndrome, and idiopathic (functional) causes participated in a multiphase study (open, double blind, and long term open) of loperamide HCl, a new single entity, oral antidiarrheal agent. In the open phase of the study, loperamide effectively relieved symptoms of diarrhea in 21 of 27 patients. The average number of stools dropped from eight in the initial relapse period to two stools after 1 month of treatment (P = 0.0001). Efficacy was confirmed in the double blind and long term open phases of the study. Four patients who were not relieved while on therapy had discontinued the drug because of abdominal cramping. No other side effects attributable to the drug were observed. Loperamide has been found to be a safe and effective agent for the treatment of chronic diarrhea.