To investigate the effects of air pollution on human health, we determined serum concentrations of complement components C3c and C4 in 1037 children who lived in 4 communities with different levels of air pollution in Japan. Serum levels of C3c and C4 were higher in children who lived in Osaka, which had a high level of air pollution, than in children who lived in areas of low air pollution. In boys, both C3c and C4 levels were increased significantly as concentrations of air pollution increased in the communities. In girls, however, the relationship was not significant. Serum levels of C3c and C4 did not differ with respect to asthma or wheezing. These findings suggest that serum C3c and C4 levels in children reflect the effects of exposure to air pollutants in urban districts. Boys appeared to be more susceptible to the effects of air pollution than girls.