Diarrhoea is the most common illness affecting travellers to developing countries. Our study was designed to compare the efficacy of a single 500 mg dose of ciprofloxacin with placebo for treatment of acute diarrhoea in travellers. British troops who were within their first 8 weeks of deployment in Belize and who presented within 24 h of the onset of diarrhoea, were randomized to receive either ciprofloxacin 500 mg or placebo. Every subject recorded the number and consistency of stools and presence of any other associated symptoms for 72 h or until recovery. Of 88 subjects enrolled, 83 were evaluable, of whom 45 received ciprofloxacin and 38 placebo. Groups did not differ with regard to duration or severity of diarrhoea at randomisation. Mean (SE) duration of diarrhoea, as assessed by time to the last liquid and last unformed stool, was reduced from 50.4 (4.5) h and 53.5 (4.4) h, respectively, in the placebo group to 20.9 (3.4) h and 24.8 (3.8) h in those receiving ciprofloxacin (p < 0.0001). Mean number of liquid stools was reduced from 11.4 (1.2) in the placebo group to 5.0 (0.7) in the ciprofloxacin-treated group (p < 0.0001). The cumulative percentages of subjects with no unformed stool after 24 h, 48 h, and 72 h were, respectively, 64%, 82%, and 93% in the ciprofloxacin group and 11%, 42%, and 79% in the placebo group (p < 0.0001, p < 0.001, and not significant, respectively). A single 500 mg dose of ciprofloxacin was an effective empirical treatment for reducing the duration and severity of diarrhoea in travellers. The regimen should maximise compliance and reduce the cost and duration of therapy.