The large number of subjects increases the power of the analysis in this study. Further, several aspects of homogeneity were advantageous for the purpose of our analysis. For example, subjects represented all public elementary and junior high schools in Tokorozawa, and can be considered representative of most 6- to 15-year-old children in this area. Important confounding factors were obtained in this study, which was not the case in previous reports. Adjustment for these confounding factors is expected to clarify the independent association between asthma and breast feeding.