Background: Respiratory syncytial virus (RSV) is the major cause of acute lower respiratory infection (LRTI) in infants and children. The association between climate factors and RSV epidemics has been suggested, but the relationship of RSV activity with air pollution is unclear. We aimed to investigate the association of meteorological conditions and air pollution with RSV activity in Seoul, Korea. Methods: Between January 2005 to December 2012, a total of 10,046 nasopharyngeal swab specimens from children aged 0-14 years who admitted the hospital with acute LRTI were tested for RSV antigens using direct immunofluorescence kit. Meteorological data (mean temperature, precipitation, wind speed, and relative humidity) and air pollutant levels including PMsub＞10/sub＞, PMsub＞25/sub＞, NOsub＞2/sub＞, Osub＞3/sub＞, SOsub＞2/sub＞, and CO in Seoul during the study period were collected from National monitoring system. The correlation between monthly incidence of RSV and climate factors and air pollutants levels was analyzed. Results: RSV infection mainly occurred from October to February and showed peak in November. RSV incidence had negative correlation with the mean temperature, precipitation, and relative humidity. Regarding air pollutants, RSV activity was positively correlated with NOsub＞2/sub＞, SOsub＞2/sub＞, and CO, whereas negatively correlated with Osub＞3/sub＞ which was the only significant factor in the multivariate analysis. By season, RSV epidemics had negative correlation with mean temperature and Osub＞3/sub＞ in spring, whereas in summer it showed positive correlation with mean temperature and relative humidity, but negative correlation with NOsub＞2/sub＞, Osub＞3/sub＞, SOsub＞2/sub＞, and CO. In autumn, RSV activity showed negative correlation with mean temperature, relative humidity, and Osub＞3/sub＞, but positive correlation with wind speed, NOsub＞2/sub＞, SOsub＞2/sub＞, and CO. In winter, only relative humidity was positively associated with RSV incidence. Conclusion: Meteorological factors (mean temperature, relative humidity, and precipitation) and air pollutant levels (NOsub＞2/sub＞, SOsub＞2/sub＞, CO, and Osub＞3/sub＞) were associated with RSV activity, especially Osub＞3/sub＞ was the most significant factor related to RSV infection.