We studied 240 children with asthma who were themselves nonsmokers and had been referred consecutively to our clinic. They were aged 6 to 17 years. The severity of asthma was assessed by symptom score, by spirometry, and, in those who could perform the test reliably, by histamine bronchial challenge test. Those who reported having had a chronic or chronically relapsing itchy rash in characteristic locations were recorded as having had atopic dermatitis. Multiple analysis of variance revealed that children whose mothers smoked had significantly more severe asthma (p less than 0.001) but that atopic dermatitis had no apparent effect on the severity of asthma, either in its main effect (p = 0.71) or in its interaction with maternal smoking (p = 0.66). Although our previous study indicates that smoking mothers' children are more likely to develop asthma if they have had atopic dermatitis than if they have not, the severity of asthma does not appear to be associated with a history of atopic dermatitis. In smoking mothers' children the asthma was just as severe in those who had not had atopic dermatitis as in those who had.