Our population of children was obtained from the asthma index families using the following inclusion criteria: ( 1 ) age 8 and 15 yr;( 2 ) child had never smoked; ( 3 ) mother had never smoked; ( 4 ) siblings 20 yr of age had never smoked; ( 5 ) coal was not used for either heating or cooking in the home. Children with formerly (but not currently) smoking fathers were also excluded from the analysis. Of 1,904children age 8 to 15 yr who were available from the asthma index families, 1,718 met our inclusion criteria (860 girls and 858 boys).