OBJECTIVE Viruses are the most common causes of respiratory tract infection in children. We investigated the aetiologies and the epidemiological features of acute viral respiratory tract infections in Korean children. METHODS We tried to isolate respiratory syncytial virus (RSV) and parainfluenza virus from January 1994, influenza virus from February 1995, and adenovirus from April 1996 through August 1998, and identified the isolated viruses by indirect immunofluorescence (IF) staining in the children hospitalized with acute respiratory tract infections (ARTI). RESULTS Virus was identified in 360 of 1389 (25.9%) nasopharyngeal aspirates cultured. Of a total of 392 viruses, 164 (41.8%) RSV, 90 (23%) parainfluenza virus, 66 (16.8%) influenza A virus, 54 (13.8%) adenovirus, and 18 (4.6%) influenza B virus were cultured, including cases in mixed viral infections. The male to female ratio of the culture-positive patients was 2:1, and the proportions of the patients aged >6 months, 6-11 months, 1, 2, 3, 4, 5, 6-7, 8-9, and >10 years were 22.5, 29.5, 25.7, 9.5, 3.8, 3.8, 1.7, 1.7, 1.2, and 0.6%, respectively. The major clinical diagnosis was bronchiolitis for RSV, croup for parainfluenza virus, and pneumonia for adenovirus and influenza virus. Infections by RSV, parainfluenza virus, and influenza virus occurred in annual epidemics, and infections by adenovirus occurred annually with or without epidemics. There were somewhat larger epidemics by adenovirus and influenza virus in May to July 1996 and March to June 1997, respectively. CONCLUSIONS Viral agents are one of the main aetiologies and the main causes of admission in Korean children with ARTI.