Objective: To assess the association of air pollution and school absenteeism among elementary students. Design: Time-series analysis of air pollution and school absenteeism data with controlling for long-term trends, seasonality, day of the week, and holiday as well as meteorologic variables. Setting: School absenteeism data for the period from March 2, 1996, to December 22, 1999, were collected from student attendance reports of one elementary school in Seoul, Korea. Main Outcome Measures: The number of daily illness-related absences was analyzed against the daily levels of air pollution by generalized additive Poisson regression. The relative risks of absenteeism for air pollution exposure of interquartile ranges (the range from the lowest 25% of the value to the lowest 75% of the value) on absenteeism were calculated on the same day. Results: Exposure to air pollutants such as particulate matter of 10 μm or less in aerodynamic diameter (PM10), sulfur dioxide, and ozone, was associated with illness-related absenteeism. The estimated relative risks were 1.06 (95% confidence interval, 1.04-1.09) per 42.1-μg/m3 increase in PM10, 1.09 (95% confidence interval, 1.07-1.12) per 5.68-part per billion increase in sulfur dioxide, and 1.08 (95% confidence interval, 1.06-1.11) per 15.94-part per billion increase in ozone. There was no significant relationship between nitrogen dioxide level and illness-related absenteeism. Conclusion: Air pollution is associated with illness-related absences among elementary students.