Nasopharyngeal secretions and throat-swab specimens from 809 children less than 6 years old with acute respiratory infection were examined by culture and indirect immunofluorescence for the presence of virus or viral antigen. Blood was cultured for the presence of bacteria in selected cases of lower respiratory infection (LRI); pleural fluid also was cultured in cases of empyema. Viruses were detected in 163 (49%) of 331 children with LRI. Respiratory syncytial virus (RSV) was the commonest agent isolated (106 children). Other viruses isolated included parainfluenza viruses (36 children), adenoviruses (12), and influenza viruses (five). Outbreaks of infection due to RSV occurred during August through October. Pneumonia was the commonest LRI encountered (178 children). Among children with pneumonia, viruses were detected in 65 (37%) of 178 children, and bacteria were isolated from 27 (18%) of the 147 children for whom blood cultures were done. Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus were the common bacterial pathogens isolated. In cases of empyema and pyopneumothorax, S. aureus was the commonest organism isolated. There were 116 children with bronchiolitis, 83 (72%) of whom had viral infections; the majority of these children (81%) had RSV infection. Croup was uncommon (eight cases) and was caused mainly by parainfluenza viruses.