PURPOSE: This is to analyze epidemic and clinical features of acute respiratory tract infections (ALRTI) by respiratory syncytial virus (RSV) and parainfluenza virus (PIV), which are considered to be the major respiratory pathogens in children. METHODS: Nasopharyngeal aspirates were collected from 515 patients with ALRTI hospitalized at Hangang Sacred Heart Hospital between Apr. '94 and Oct. '96. Nasopharyngeal aspirates were inoculated to HEp-2 cell, MDCK cell, and LLC-MK2 cell for the cultivation of viruses. The cultivated cells were observed for cytopathic effect on the 4th and 10th day. In case cytopathic effect was seen, indirect IF was done by using monoclonal antibodies for RSV and PIV 1, 2, 3 types. RESULTS: 1) The RSV and/or PIV have been identified in 150 out of 515 patients (29.1%), which break down into 86 patients (cultured only RSV), 46 patients (only PIV), 18 patients (both RSV and PIV). 2) ALRTI caused by RSV and PIV occurred mostly in the first two years of life (93%). The ratio of male to female patient was 2.1:1 and 74% of ALRTI was found in the patients with no history of underlying diseases. 3) RSV infection occurred mostly late autumn and winter, while PIV infection occurred all the year round, mostly in May. 4) RSV infection caused bronchiolitis in 67%, pneumonia in 28%, croup in 4%, while PIV infection caused bronchiolitis in 41%, croup in 37%, pneumonia in 20% clinically. And combined infection caused bronchiolitis in 67% and pneumonia in 28%. 5) The WBC count was normal in 49% of the RSV, 35% of the PIV, and 56% of the combined infection. The ESR was normal in 71% of the RSV, 59% of the PIV, and 79% of the combined infection. The CRP was normal in 83% of the RSV, 66% of the PIV, and 89% of the combined infection. 6) The following non-respiratory signs and symptoms were detected: diarrhea, increased AST/ALT, conjunctivitis, febrile seizure, exanthem, acute otitis media, enanthem. 7) The major radiologic features of RSV were normal in 54%, hyperaeration in 28%, peribronchial infiltration in 8%, and consolidation in 8%. PIV infection shows normal in 30%, hypopharyngeal dilatation in 33%, hyperaeration in 17%, and peribronchial infiltration in 4%. 8) The hospitalized period was 7.1 days on the average. None of them was died or mechanically ventilated due to ALRTI. CONCLUSIONS: RSV and PIV are considered to be the major causes of ALRTI. Diagnosis and treatment of RSV and PIV infections will be improved through further epidemiologic or clinical studies.