The aim of this study was to determine the viral agents associated with acute lower respiratory infections (ALRI) in young children. During a 2-yr period, 204 nasopharyngeal aspirates (NPA) from children under 4 yr of age living in an orphanage and exhibiting febrile ALRI were studied by both indirect immunofluorescence (IIF) and isolation in four cell lines. NPA cell smears as well as tissue culture cells exhibiting cytopathic effect (CPE) or hemadsorption were stained by IIF for respiratory syncytial virus (RSV), adenovirus, influenza A and B, and parainfluenza 1 and 3. Viral etiology was demonstrated in 21.2% of acute respiratory infection cases. The most frequently detected virus was RSV (53.5% of viral positive diagnoses), followed by unidentified viruses (18.6%), adenovirus (13.9%), influenza A (7%), and parainfluenza 3 (4.7%). The most common clinical entities were: bronchitis, 46.1%; pneumonia, 24%; bronchiolitis, 22%; and multifocal pneumonia, 8%. Malnourishment was found in 56% of children with ALRI, whereas 50% was found among total orphanage population. The 3 to 8-month-old age group accounted for half of all ALRI cases. Viral etiology was shown for 26.5% of patients with pneumonia, 22% with bronchitis, and 16% with bronchiolitis. RSV and adenovirus occurred in fall and winter, while parainfluenza 3 was detected in early spring. In the two fatal cases observed, histologic lesions were compatible with adenovirus infection, but this virus could be isolated from the lung in only one case.