A randomized double-blind controlled trial was carried out to compare the effectiveness and side-effects of cefaclor with those of amoxicillin in the treatment of 34 patients hospitalized because of acute bacterial pulmonary infection. Cefaclor and amoxicillin were given three times daily before meals in a dosage of 500 and 750 mg respectively for seven days. The cure rate was 94% in the group of patients treated with cefaclor and 89% in the group treated with amoxicillin. According to all the parameters used to assess outcome, no significant difference was found between the two antibiotics tested. In this study, cefaclor and amoxicillin appeared to be of equal value in the treatment of bacterial pneumonia or bronchopneumonia.