Background: Although the use of the antimicrobial, trimethoprim-sulfamethoxazole, in combination with the antisecretory and antimotility agent, loperamide, has been shown to be efficacious in the treatment of traveler's diarrhea, the use of fluoroquinolone antimicrobials in combination with loperamide has less support in the literature. The present study was designed to compare the efficacy of ofloxacin versus ofloxacin plus loperamide in the treatment of acute traveler's diarrhea. Method: This prospective, randomized, evaluator-blinded treatment trial was conducted in Guadalajara, Mexico, during the summers of 1992-1994. Adults newly arrived in Mexico from the United States who developed acute diarrhea of less than 2 weeks' duration were randomized to receive orally either: A) ofloxacin, 400 mg once; B) ofloxacin, 200 mg twice a day for six doses; or C) ofloxacin, 400 mg once, plus loperamide, 4 mg once followed by 2 mg after each loose stool, not to exceed 16 mg per day, for 3 days. The duration of illness was the number of hours elapsed from the beginning of therapy to the passage of the last unformed stool. Results: Ofloxacin and loperamide were well tolerated. Combination therapy with single dose ofloxacin plus loperamide was significantly more efficacious in reducing the duration of diarrhea than single dose ofloxacin or ofloxacin given for 3 days (p <.00001). Furthermore, combination therapy was more efficacious when enterotoxigenic Escherichia coli (ETEC) was the pathogen (p <.01) or when no pathogen was isolated (p <.001). Sixty-three percent of subjects passed no further unformed stools after the initial doses of combination therapy, and 91% were well by the end of the first 24 hours. Conclusions: The combined use of a single dose of ofloxacin with loperamide is safe and more efficacious in the treatment of traveler's diarrhea than use of ofloxacin alone.