Loperamide hydrochloride was compared with bismuth subsalicylate for the treatment of acute nondysenteric travelers' diarrhea in 219 students visiting seven countries in Latin America. Subjects whose condition was not improved with therapy could elect to take trimethoprim-sulfamethoxazole. Persons receiving loperamide passed fewer unformed stools when compared with the bismuth subsalicylate group during the first four hours of therapy, from four to 24 hours, and from 24 to 48 hours after therapy was initiated. Among subjects with disease due to enterotoxigenic Escherichia coli, Shigella sp, other pathogens, and unknown agents, fewer unformed stools were passed by the loperamide-treated subjects than the bismuth subsalicylate-treated subjects for all time periods studied. No significant prolongation of disease was seen in subjects with shigellosis treated with loperamide. Eight of the loperamide-treated subjects experienced constipation compared with one in the bismuth subsalicylate-treated group; otherwise, there was no difference in minor side effects experienced between both treatment groups. We conclude that loperamide is a safe and effective alternative to bismuth subsalicylate for the treatment of nondysenteric travelers' diarrhea.