In a double-blind, randomized trial, 85 adult patients with acute diarrhea (more than three watery stools per day) received either 500 mg of ciprofloxacin twice daily or placebo for five days. Seventy-six patients were evaluated, 38 patients in the ciprofloxacin group (16 with Salmonella species, 19 with Campylobacter jejuni, and three with Shigella species) and 38 patients in the placebo group (21 with Salmonella species, 11 with C. jejuni, and six with Shigella species). The duration of fever in patients treated with ciprofloxacin was 1.3 days versus 3.1 days in the placebo group (p less than 0.05). The mean duration of diarrhea in the ciprofloxacin group was 1.5 days versus 2.9 days in the placebo group (p less than 0.001). The corresponding numbers in patients with salmonellosis were 1.9 versus 3.4 days (p less than 0.01). In the ciprofloxacin group, all stool culture results became negative within 48 hours of treatment. Relapse occurred in four patients with salmonellosis within three weeks after the end of treatment. In the placebo group, only four of 38 patients had negative stool culture results during treatment and results were negative in only 13 at one week after the treatment period (p less than 0.001). Modest transient elevation of serum transaminase levels was detected in three patients in the ciprofloxacin group and in two patients in the placebo group. Epigastric pain occurred in one patient, and leukopenia occurred in one patient in the ciprofloxacin group. Gastrointestinal discomfort was recorded in two patients and rash was found in one patient in the placebo group.