To quantitatively assess environmental tobacco smoke exposure from parental smoking as a risk factor for asthma or wheezing in childhood, and to derive estimates of excess asthma/wheezing lower respiratory tract illness cases attributable to this risk factor, a cross-sectional analysis of the 1987 National Medical Expenditure Survey (a national probability sample of the civilian, noninstitutionalized US population) was undertaken. The National Medical Expenditure Survey included 7,578 children and youth less than 18 years of age in a stratified cluster sampling of US households. After using logistic regression analysis to control for sex, race/ethnicity, region of residence, population density, poverty status, maternal educational level, family size, and father's current smoking status, children whose mothers smoked at the time of the survey were more likely than children of nonsmoking mothers to experience wheezing respiratory illness (odds ratio = 1.36, 95% confidence interval 1.14-1.62). The association was greatest among children less than 2 years of age. The authors' estimate of the attributable risk in the US population indicates that maternal smoking is responsible for approximately 380,000 excess cases of childhood asthma/wheezing lower respiratory tract illness or 7.5% of the total number of such symptomatic children.