OBJECTIVE: Our aim was to compare the efficacy and safety of rifaximin, a virtually nonabsorbed antibiotic, 600 and 1200 mg per day, with placebo in patients with travelers' diarrhea. METHODS: This was a multicenter, 1:1:1 randomized, parallel-group, double-blind study, conducted in Antigua, Guatemala; Guadalajara and Morelia, Mexico; and the coast of Kenya north and south of Mombasa. Adult patients with acute travelers' diarrhea were recruited; exclusion criteria included primarily medication that could influence the outcome. Subjects were treated for 3 days, three times daily; follow-up lasted 5 days. For each 24-h period, the subjects completed a diary card. Pre- and posttreatment stool, blood, and urine samples were assessed. RESULTS: Among the 380 volunteers, median time to the last unformed stool was 32.5 and 32.9 h in both rifaximin groups, compared with 60.0 h with placebo (p = 0.0001). Also, secondary clinical outcome measures were favorably influenced by the active agent. No relevant side effects were reported. CONCLUSION: Rifaximin is efficacious and safe for treatment of travelers' diarrhea at daily doses of 600 mg or higher.