For a study of childhood asthma we interviewed 9,276 mothers during 1993-1994, ascertaining whether they had asthmatic children younger than 18 yr of age and asking about genetic and environmental risk factors for asthma. Independent risk factors for asthma in 7,776 children were: Hispanic and African American ethnicity, maternal history of asthma, lower socioeconomic status (SES) of the mother, and the presence of a cigarette smoker in the household. Hispanic ethnicity was also a strong risk factor for asthma in the mother. The prevalence of asthma among children of Hispanic (mainly Puerto Rican) mothers with one or more children older than 9 mo of age was 18.4%, for blacks it was 1 1.3%, and for non-Hispanic whites it was 7.4%. The marked increased risk for asthma in children of Hispanic mothers was not explained by SES or maternal age. In addition, increased risk for asthma in these children was not associated with higher reporting of environmental tobacco smoke (ETS) exposure. In this study of asthma in primarily Puerto Rican Hispanics, the risk of physician-diagnosed asthma as reported by mothers was significantly associated with Hispanic ethnicity, and it was not confounded by SES or active smoking in the home.