Asthma is one of the commonest of chronic illnesses affecting children. Parental smoking has been considered to have an effect on this. In an attempt to clarify the relationship between parental smoking and the prevalence of childhood asthma we interviewed parents of 97.5% of the children aged 3-11 years registered with a large urban British general practice. We found a lifetime prevalence of asthma of 19.6%, 23.2% of boys and 15.9% of girls. Asthma was more common, 37.6%, in children who also had eczema. Parental smoking appeared to increase the prevalence of asthma. This was more marked when both parents smoked.