Secondary bacterial infection was studied on 231 children admitted with Respiratory Syncytial virus (RSV) infection in the 10 years since 1987. Of the 231 children, 56 (24.2%) had dual bacterial infection possibly due to secondary bacterial invasion. The diagnoses of bacterial disease were sepsis (2), pyothorax (2), pneumonias (41), otitis media (7), nasopharyngitis (2) and urinary tract infection (2). Dual bacterial infections were more frequent in infants and children over 6 months than in infants younger than 6 months. The main etiologic agents were Staphylococcus aureus and enteric gram-negatives in infants, and Haemophilus influenzae, Streptococcus pneumoniae, beta streptococci and Branhamella catarrhalis in children over 1 year. The incidence of secondary bacterial infection was compared according to the usage of antibiotics just before admission. Patients who had been administered with penicillins or macrolides before admission had a significantly higher percentage of secondary bacterial infection (21/56, 37.5%) than those of no previous antibiotic therapy (11/64, 17.2%, p less than 0.025). The results indicate that the RSV infection itself sometimes predisposes to secondary bacterial infections, but indiscriminate use of antibiotics further increases the risk of secondary bacterial infections.