This study is a double-blind, placebo-controlled, randomised clinical trial to evaluate the clinical and microbiological efficacy and safety of single dose ofloxacin for acute diarrhoea. Eligible patients were 16 years of age or older with a history of acute diarrhoea lasting no more than 48 h; 117 patients were randomised and 97.4% (114/117) were evaluable for efficacy. Of these, 58% were suspected to have ingested contaminated foods. Enteric pathogens were isolated in 61.5% of the patients, Salmonella enteritidis being reported in 87.5%. The patients received either a single 400 mg dose of ofloxacin, or placebo. The average duration of diarrhoea was 2.56 ± 2.21 days in the ofloxacin group and 3.41 ± 2.5 in the placebo group (P = 0.117). The average duration of fever was 0.63 ± 0.95 days in the ofloxacin group and 105 ± 0.96 in the placebo group (P = 002). Symptoms remained unchanged for more than 48 h in only 7% of the patients who received ofloxacin, compared with 12% in the placebo group (P = 0-485). Only 32% of patients in the ofloxacin group remained culture positive after 48 h compared with 59% in the placebo group (P = 00018). These represent a relative risk reduction (RRR) for stool clearance of 45-5% and absolute risk reduction (ARR) of 27% (95% Cl, 8-44-7), with a number of patients needed to treat (NNT) of 3-7 (95%, 2-7-11-3). After 15 days, 23-3% of patients in the ofloxacin group had a positive culture compared with 28-9% in the placebo (P = 0-63). This represents an RRR of 19%, an ARR of 5-6% and a NNT of 17-8. Adverse events in the ofloxacin group were observed in only one patient who reported headache and in one patient in the placebo group who developed a rash. In summary, empirical treatment with a single dose of ofloxacin in acute diarrhoea did not reduce the intensity or duration of symptoms (except possibly length of fever). It was notable however that stool cultures became negative for S. enteritidis by 48 h, with no relapse after 2 weeks of follow-up.