We reviewed data collected between January 1987 and December 1989 on the etiology of acute respiratory infections (ARI) among 827 children in two low-income communities and a hospital in Rio de Janeiro. Respiratory syncytial virus was identified in 38% of cases of ARI, influenza A virus in 1%, parainfluenza 3 virus in 1%, and multiple viruses in 1%. Respiratory syncytial virus was most prevalent among hospitalized children, with seasonal increases in the late fall and winter. The principal bacterial pathogens were Staphylococcus aureus, gram-negative bacteria, Streptococcus pneumoniae, and alpha-hemolytic streptococci. Specimens that were most often positive were pleural fluid (46%) and specimens from other normally sterile sites (24%); normally sterile sites included the CSF, trachea, and lungs. Urine counterimmunoelectrophoresis for S. pneumoniae and Haemophilus influenzae polysaccharide antigens was positive in 3% and 2% of cases, respectively. Pharyngeal cultures yielded low numbers of S. pneumoniae and H. influenzae organisms and higher numbers of gram-negative bacteria. This study demonstrates the high incidence of ARI (4.5 cases per 100 child-weeks) among children in Rio de Janeiro and the high morbidity associated with the illness (ARI is responsible for 25%-50% of all pediatric hospitalizations) and the fact that continued attention must be paid to both viral and bacterial agents of ARI.