A total of 1429 Saudi children of either sex and under 5 years of age who were admitted to King Khalid University Hospital, Riyadh during a three year period (April 1993-March 1996) with complaints suggestive of acute respiratory tract infections (ARTI) were investigated for viral aetiology of the infection. Viruses could be detected in 522 (37 per cent) cases with respiratory syncytial virus (RSV) the most commonly detected (79 per cent) followed by parainfluenza type 3 (8 per cent). Detection of influenza A, B and adenoviruses accounted for 6 per cent, 3 per cent and 2 per cent respectively. Except for parainfluenza virus type 3 the peak of activity of the respiratory viruses was during the winter months (October-February). Parainfluenza virus type 3 could be detected all year round but epidemics can occur in the hottest months of the year (June-August) when the temperature can exceed 40 degrees C. Association between clinical manifestation, type of causative agent, and age was evaluated in 137 infected Saudi children in the first year of the study (April 1993-March 1994). The majority of our cases presented with bronchiolitis (58 per cent) while only 26 per cent had bronchopneumonia. There was a significant association between bronchiolitis and lower age groups (0-6 months), with RSV as the major causative agent of bronchiolitis cases (88 per cent).