: the presence of an asthma patient who was at least 8 yr old;availability of both parents for the study; the presence of at least one sib-ling, at least 8 yr old; and no more than one parent with asthma. Diagno-sis of asthma was made by local physicians according to the following criteria: the patient had a history of repeated onset of wheeze with dysp-nea, but was without symptoms between two events; and the obstructivesymptoms could be improved significantly by a bronchodilator.Our population of children was obtained from the asthma indexfamilies 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 heat-ing or cooking in the home. Children with formerly (but not cur-rently) smoking fathers were also excluded from the analysis. Of 1,904children age 8 to 15 yr who were available from the asthma index fam-ilies, 1,718 met our inclusion criteria (860 girls and 858 boys).