According to a substantial literature, passive smoking by children is associated with an increased incidence of lower respiratory illness and diminished pulmonary function. The relationship between passive smoking and childhood asthma, however, is not clear. Data from the Child Health Supplement to the 1981 National Health Interview Survey were analyzed with information about 4331 children aged 0 to 5 years to study the relationship between maternal smoking and (1) the prevalence of childhood asthma, (1) the likelihood of taking asthma medication, (3) the age of onset of children's asthma, and (4) the number of hospitalizations among children with and without asthma. An odds ratio for asthma of 2.1 was shown by multivariate logistic regressions among children whose mothers smoke 0.5 packs of cigarettes or more per day compared with children of nonsmokers (P = .001). In similar analyses maternal smoking of 0.5 packs per day was identified as an independent risk for children's use of asthma medications (odds ratio 4.6, P = .0006) and for asthma developing in the first year of life (odds ratio 2.6, P = .0006). Maternal smoking is also associated with increased numbers of hospitalizations by its association with an increased risk of asthma as well as by contributing to hospitalizations independently of a child having asthma. Among children with asthma, however, maternal smoking is not associated with increased numbers of hospitalizations. It was concluded that maternal smoking is associated with higher rates of asthma, an increased likelihood of using asthma medications, and an earlier onset of the disease.(ABSTRACT TRUNCATED AT 250 WORDS)