To evaluate the efficacy of early treatment with erythromycin on the duration of fecal excretion and of diarrhea associated with Campylobacter jejuni, 170 patients, age 3 to 60 months, were randomly assigned in a double-blind fashion to receive either erythromycin ethyl succinate or placebo immediately after being seen at Cayetano Heredia Hospital because of acute dysentery. The groups' pretreatment characteristics were comparable. Of the 30 patients with stools positive for C. jejuni, 12 were in the placebo group and 16 in the treatment group. After 2 days of treatment, none of the patients in the placebo group and 36% of those in the erythromycin group had normal stools (P less than 0.05). After 5 days of treatment, 50% of the patients in the placebo group and 93% of those in the erythromycin group had normal stools (P less than 0.02). Fecal excretion of the organism continued significantly longer in the placebo group (P less than 0.01). There were no treatment failures in the treatment group compared with five (42%) in the placebo group (P less than 0.01). Thus, early administration of erythromycin significantly reduced the duration of both diarrhea and fecal excretion of the organism in infants and children with acute dysentery associated with C. jejuni.