The occurrence of respiratory syncytial virus (RSV) infection among young children hospitalized with lower respiratory tract illness, at King Khalid University Hospital in Riyadh, was examined during the autumn-winter season between September 1991 and February 1992. Sixty-nine cases were diagnosed by immunofluorescent antibody staining of viral antigen in nasopharyngeal aspirates from 127 children, constituting 54 per cent of these patients. Virus culture was attempted only in a few cases, yielding two isolates. Most children were < 1 year of age (median 2 months). Bronchiolitis and bronchopneumonia were the major diagnoses on admission. Hospitalization was for an average of 5 days (range 1-36 days). Treatment was supportive but most children received antibiotic therapy. There was no mortality. Few other bacterial or viral pathogens could be identified from RSV-positive or -negative patients. These results indicate that, during the season of infection, RSV may be the main pathogen of lower respiratory tract illness in hospitalized young children in this region.