Viruses are considered major causes of acute respiratory tract infections among children under 5 years old. In this study we investigated the prevalence of three respiratory viruses--respiratory syncytial virus (RSV), influenza virus (INF) and adenovirus (ADV)--among hospitalized children with acute viral lower respiratory tract infections (LRTIs). Nasopharyngeal aspirates were collected from children under five who had been hospitalized for LRTIs. The clinical data, including demographic data (age and sex), vital symptoms and signs at admission, duration of fever, duration of hospitalization, chest X-ray findings and outcome were considered. All inpatient specimens were tested by reverse transcriptase-polymerase chain reaction (RT-PCR) for RSV and the INF-A, INF-B and parainfluenza viruses and by polymerase chain reaction (PCR) for ADV. Out of those from 232 patients, 58 (25%) specimens were positive for either RSV, INF or ADV. The most predominant pathogens were RSV (40 cases, 17.2%), followed by INF (10 cases, 4%; including 8 type A and 2 type B) and ADV (8 cases, 3.4%). A total of 32 (55.1%) viral cases were identified in the spring, followed by 19 (32.7%) in the autumn and 7 (12%) in the winter. There was no significant correlation between clinical symptoms and the individual virus detected. In our study, RSV and INF were the two most common causes of LRTIs. These data are helpful for guiding the development of further vaccines as well as the use of antiviral drugs. Further studies will be needed to investigate other respiratory viruses such as parainfluenza, human metapneumovirus and rhinovirus.