• We attempted to determine the causative bacterial pathogens of impetigo in children in our area, to compare the effectiveness of three frequently used oral antimicrobial treatment regimens, and to correlate the antimicrobial sensitivity of the bacterial isolates with clinical responses to treatment. Seventy-three children with impetigo were randomly assigned to receive penicillin V potassium or cephalexin monohydrate, both administered in dosages of 40 to 50 mg/kg per day, or erythromycin estolate administered in a dosage of 30 to 40 mg/kg per day. All drugs were given in three divided doses for 10 days. Treatment failure was defined as persistence of lesions 8 to 10 days after initiation of drug therapy as determined by examiners blinded to the treatment therapies. Forty-five (62%) cultures showed Staphylococcus aureus only, 14 (19%) showed S aureus and group A β-hemolytic streptococci, six (8%) showed group A β-hemolytic streptococci only, and eight (11%) showed no growth or other organisms. Treatment failure occurred in six (24%) of 25 patients treated with penicillin V, one (4%) of 25 patients treated with erythromycin estolate, and no patients treated with cephalexin. We conclude that S aureus is the most common cause of impetigo in children in our study population, that cephalexin is the most effective treatment, that erythromycin estolate is nearly equally effective and may be preferred on a cost-effectiveness basis, and that penicillin V is inadequate for treatment of this infection.