Norfloxacin, an oral fluoroquinolone (dose 400 mg daily), was compared to a placebo in a double blinded randomized trial for the prophylaxis of travelers' diarrhea. The study was of U.S. Navy and Marine Corps personnel on shore leave in Alexandria, Egypt. A total of 222 subjects were available (105 norfloxacin, 117 placebo). In the placebo group, 26% (30/117) developed acute diarrhea vs. 2% (2/105) in the norfloxacin group. There were no significant side effects in either group.