The efficacy of ciprofloxacin was compared with that of trimethoprim-sulfamethoxazole in a placebo-controlled trial of the 5-day treatment of acute diarrhea among 181 adults recently arrived in Guadalajara, Mexico. Both antimicrobial agents were significantly (p less than 0.0001) more efficacious than placebo in the treatment of diarrhea, with the average duration of diarrhea being 29, 20, and 81 hours, respectively, in the ciprofloxacin, trimethoprim-sulfamethoxazole, and placebo treatment groups. The antimicrobial agents were also more efficacious than placebo in treating diarrhea caused by enterotoxigenic Escherichia coli, invasive enteropathogens, and unknown pathogens. Both antimicrobials were effective in treating mild-to-moderate and moderate-to-severe disease, and both were well tolerated. Ciprofloxacin appears to be a logical alternative to trimethoprim-sulfamethoxazole in the initial treatment of acute travelers' diarrhea.