We elicited symptoms of atopic dermatitis (AD) and of asthma from 620 children who were themselves nonsmokers, were aged 1 to 17 years, and had been consecutively referred to an allergy clinic. A histamine bronchial-challenge test revealed bronchial hyperresponsiveness in 95% of the children who had a history of wheezing or asthma and who could perform the test reliably, indicating that most of them did indeed have asthma. Children with a history of AD were much more likely to have asthma if the mother was a smoker than if she was a nonsmoker (79% versus 52%; p = 0.001). Similarly, if AD was found on examination, the percentages with asthma were 74% and 44%, respectively. By contrast, the children with no history of AD had asthma as frequently if the mother was a nonsmoker (42%) as when she was a smoker (40%). In children with AD, the prevalence of asthma was greater in both boys and girls when the mother was a smoker, but only in boys when the father was a smoker. Multiple logistic regression confirmed that the risk of asthma was greatly increased when the child had both AD and a mother who smoked.