The objective of this study is to assess whether exposure to air pollutants is associated with postneonatal infant death, using a time-series methodology, between 1999 and 2003 in Seoul, Korea. We investigated the short-term effects of air pollution for 548,725 live births during the study period. The daily count of postneonatal infant deaths from all causes and from SIDS (sudden infant death syndrome) by birth order was analyzed by a Generalized Additive Poisson model, with controlling for the effects of seasonal trends, air temperature, relative humidity, barometric pressure, and day of the week as covariates. During the study period, we observed 1164 deaths from all causes and 83 deaths from SIDS. We did not find significant associations between daily mortality and ambient levels of air pollutants except CO and NO₂. The estimated relative risk of postneonatal infant death from all causes was 1.17 (95% CI = 1.04-1.32) and 1.16 (95% CI = 1.03-1.29) by IQR (interquartile range) for CO and NO₂respectively. Also, we observed no clear trend of the mortality effects of air pollution by birth orders. In conclusion, our findings suggest that air pollutants (except ozone) should be positively associated and influenced adversely postneonatal infant death from all-cause and SIDS although they are not statistically significant. The objective of this study is to assess whether exposure to air pollutants is associated with postneonatal infant death, using a time-series methodology, between 1999 and 2003 in Seoul, Korea. We investigated the short-term effects of air pollution for 548,725 live births during the study period. The daily count of postneonatal infant deaths from all causes and from SIDS (sudden infant death syndrome) by birth order was analyzed by a Generalized Additive Poisson model, with controlling for the effects of seasonal trends, air temperature, relative humidity, barometric pressure, and day of the week as covariates. During the study period, we observed 1164 deaths from all causes and 83 deaths from SIDS. We did not find significant associations between daily mortality and ambient levels of air pollutants except CO and NO₂. The estimated relative risk of postneonatal infant death from all causes was 1.17 (95% CI = 1.04-1.32) and 1.16 (95% CI = 1.03-1.29) by IQR (interquartile range) for CO and NO₂respectively. Also, we observed no clear trend of the mortality effects of air pollution by birth orders. In conclusion, our findings suggest that air pollutants (except ozone) should be positively associated and influenced adversely postneonatal infant death from all-cause and SIDS although they are not statistically significant.