Although most strains of Campylobacter jejuni are susceptible in vitro to erythromycin and the drug has been recommended for treatment of campylobacter enteritis, prospective controlled trials have not been done. Erythromycin (250 mg 6-hourly for adults and 40 mg/kg daily for children) has been compared with placebo in a double-blind trial of 5-day therapy for acute campylobacter enteritis. The mean number of days of illness at onset of therapy was 5.6 for the treatment group (n = 15) and 6.5 for the placebo group (n = 14). There were no differences in temperature, symptoms, and stool characteristics between the two groups. The mean duration of unformed stools after treatment was 4.1 days in the erythromycin group and 3.5 days in the placebo group. Fever, when present, abated within 48 h in all but two patients in each group. Follow-up faecal cultures 2 days after completion of therapy, however, were negative in all 15 of the erythromycin-treated patients compared with 6 out of 14 controls. Thus erythromycin promptly eradicates C. jejuni from the faeces but does not alter the natural course of uncomplicated campylobacter enteritis when therapy begins 4 or more days after the onset of symptoms.