In a prospective hospital-based study of 328 children under 5 years of age with acute lower respiratory infections, 114 (35%) were diagnosed to have acute bronchiolitis. Of them, 87 (76%) were less than 1 year and 107 (94%) were less than 2 years of age. Signs of severe lower respiratory infections, namely tachypnea (respiratory rate greater than 50/min) and subcostal retraction, were present in 95% and 93%, respectively. Of 88 children of whom roentgenographs were taken, 30 (34%) had evidence of pneumonia. No clinical signs discriminated between those with and without pneumonia. By culture or immunofluorescence antigen detection, viruses were found in 81 (71%) children with bronchiolitis; respiratory syncytial virus was the most common agent, found in 65 (57%). Parainfluenza viruses were the next most common, found in 12 (11%). Most cases of bronchiolitis occurred in outbreaks during the rainy months of August through November, coinciding with respiratory syncytial virus outbreaks. Although bacterial culture of blood was done in 56 children, no respiratory pathogen was isolated. In one child with bronchiolitis and consolidation, postmortem lung aspirate yielded Staphylococcus aureus. Thus, bronchiolitis is primarily a viral syndrome in this tropical region, just as it is in temperate regions. Eight (7%) children died (all were infants); 5 had roentgenographic pneumonia and the remaining had other abnormalities contributing to death; all had been treated with antibiotics. Since one third of lower respiratory infections are bronchiolitis, and among infants under 1 year of age bronchiolitis comprises 47% of all lower respiratory infection cases, criteria for antibiotic management must take into account the availability of roentgenographic investigation.