The recommended treatment for mild acute respiratory infections (ARI) in children is supportive care only, but many physicians, especially in developing countries, continue to prescribe antibiotic treatment because they believe it prevents progression to more severe ARI. To find out whether ampicillin treatment conferred any benefit over supportive care alone, a randomised, controlled trial was carried out among 889 children (under 5 years) with mild ARI in Indonesia. 447 were randomly allocated ampicillin (25-30 mg/kg body weight three times daily for 5 days) plus supportive care (continued breastfeeding, clearing of the nose, and paracetamol to control fever); 442 were allocated supportive care only. The treatment groups were almost identical after randomisation in terms of age, sex, level of parental education, history of measles immunisation, and fever. After 1 week the percentages cured were nearly identical (204 [46%] ampicillin; 209 [47%] control), as were the percentages of cases progressing to moderate ARI (56 [13%] vs 53 [12%]). The effect of treatment was not modified by age, sex, measles immunisation status, or the educational level of the parents. At the 2-week follow-up, the percentages cured were 62% (277) in the ampicillin group and 58% (256) in the control group; 14% of both groups had progressed to moderate ARI; and 24% (107) and 28% (123), respectively, still had mild ARI. None of the differences in outcome between the ampicillin and control groups was statistically significant. Thus, ampicillin plus supportive care offers no benefit over supportive care alone for treatment of mild ARI in young Indonesian children.