We have studied 50 children with one parent with asthma at a mean age of 6.4 years by symptom questionnaire and performed allergy skin testing and measurement of bronchial responsiveness to methacholine in both parent and child in 29-32 cases. Ninety eight per cent of the parents were receiving medication for asthma. Fifty one per cent had visited their doctor and 20% had taken more than five days off work in the previous 12 months; 12% had been admitted to hospital during the preceding 10 years. In the children the prevalences of wheeze, shortness of breath, and cough were all about double that found in a general population survey of children of similar age. Atopy was present in 90% of parents, but the prevalence of atopy among the children was not significantly different from the children in the general population. Eczema and hay fever, however, had high prevalences of 40% and 24%, respectively. Responsiveness to methacholine (provocation dose achieving 20% fall in forced expiratory volume in one second less than 6.4 mumol) was found in 93% of parents and 45% of children, which is compatible with a large increase compared with the general population. All atopic but only 50% of non-atopic children with symptoms of asthma responded to methacholine. These findings indicate that children who have one parent with asthma have roughly double the chance of developing clinical features of asthma compared with the general population and suggests that, in these children, a causal interaction occurs between atopy and bronchial hyper-responsiveness.