Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract infections in infants,and also an important factor for hospitalization during the winter months. To determine the prevalence and importance of RSV as a cause of acute lower respiratory tract infection, we carried out a prospective study during 5 months period from November to March 1998 in 6 pediatric hospitals. A nasopharyngeal aspirate was obtained for detection of RSV in all cases. Sociodemographic data, clinical signs, diagnosis and hospital admissions were documented. During this study period, 365 young infants (51.5% male, 48.5% female) with respiratory tract infection were visited in 6 hospitals. The median age of patients was 24 months (range: 1 month to 5 years).RSV infection was found in 70 out of 365 patients (19.18%).Among the 70 children with RSV infection, 29 patients (41.42%) were under 12 months of age.The main clinical manifestations of RSV infection were cough (88.57%) and coryza (78.57%). There were no significant differences between patients who were tested positive for RSV and those who were tested negative with regard to demographic variables and clinical diagnoses. This study indicates that RSV is an important cause of respiratory tract infection in infants and young children .Distinguishing RSV from other respiratory infection is difficult because of the similarity in clinical presentation among children