Clinical efficacy of norfloxacin for treatment of traveler's diarrhea in 106 Finnish tourists vacationing in Morocco was evaluated during two different seasons. When the criteria for diagnosis of traveler's diarrhea were fulfilled, norfloxacin (400 mg) or a placebo was given orally, twice daily for 3 days. All symptoms and signs subsided sooner in the norfloxacin group. The clearest difference was observed in the duration of diarrhea: 1.2 days in the norfloxacin group vs. 3.3 days in the placebo group (P < .001). The duration of diarrhea due to particular species was as follows for the two groups: in cases due to Salmonella enterica, 1.1 vs 4.1 days (P < .01); in cases due to Campylobacter jejuni, 1.8 vs. 5.0 days (P < .01); and in cases due to enterotoxigenic Escherichia coli, 1.0 day vs. 3.1 days (P < .01). The rate of full recovery during administration of norfloxacin or a placebo was also greater among the norfloxacin recipients: 84% vs. 47% (P < .001). No significant adverse effects were reported. Norfloxacin proved to be safe and effective in therapy for traveler's diarrhea.