Respiratory syncytial virus (RSV) frequently causes bronchiolitis in young infants. RSV infection was established by culture, serologic response, immunofluorescence of secretions or lung tissue or typical histology on pathologic examination of the lungs in 10 patients who were hospitalized at Children's Medical Center, Dallas. Eight were infants 4 days to 3 months old, one was 9 months and one was 16 months old. Seven patients were afebrile but had rapidly progressive diffuse infiltrates on chest roentgenograms. Six infants admitted for surgical correction of gastrointestinal anomalies or congenital heart disease had tracheal intubation before onset of pneumonia. RSV infection was acquired in the hospital in five infants. Five patients developed chronic pulmonary changes and 8 of the 10 infants died. These observations emphasize the importance of nosocomial transmission of RSV and the frequency of fatal infection in children with congenital anomalies. They also suggest that intubation can be a predisposing factor and that RSV infection can lead to chronic pulmonary changes.