PURPOSE: Palivizumab prophylaxis has been used in the high risk groups of respiratory syncytial virus (RSV) infections, especially with the prematures, infants with chronic lung diseases or hemodynamically significant congenital heart disease. Substantial variations in timing of RSV outbreaks presents a challenge for the optimized use of palivizumab prophylaxis. This study investigates the epidemiologic characteristics of RSV associated lower respiratory tract infections (LRTI) in children, to help guide in the application of palivizumab prophylaxis in the Republic of Korea. METHODS: This was a retrospective observational study. We performed RSV culture or multiplex RT-PCR from children under 60 months of age admitted for LRTI at three hospitals in the capital area of Korea from May 2008 to April 2011. The study identified RSV infection and analyzed the RSV detection rates. RESULTS: RSV detection rate was 18.8% (1,721/9,178). The RSV season of 2008-2009 is from the second week of August to the fourth week of March and, that of 2009-2010 is from the first week of October to the third week of Apirl and that of 2010-2011 is from the third week of September to the third week of March. The RSV detection rate in preterms and low birth weight infants were significantly higher during the RSV season and non-RSV season. CONCLUSION: The RSV seasons were shown to have variations in onset, offset, and durations in each year. Physicains should determine the timing of the first and final doses of palivizumab on the basis of information about the RSV season in their own area. The real-time surveillance systems to analyze the variations of RSV seasons are necessary for the effective and economical preventions of RSV infections in high risk groups.