Data from two random population surveys are used to assess the relationship between parental smoking and the prevalence of asthma in children aged 0-17. Data from a 1977 Midwestern urbanized county indicate that, if mothers smoked, the prevalence of parent reported asthma increased from 5.0 per cent to 7.7 per cent (estimated relative risk of 1.5), and the prevalence of functionally impairing asthma increased from 1.1 per cent to 2.2 per cent (relative risk of 2.0). In a more rural Eastern county in 1980, a lower overall prevalence of asthma was noted. However, similar estimated relative risks of asthma (1.8) and functionally impairing asthma (2.4) were found to be associated with maternal smoking. Inconsistent relationships were found between the estimated prevalence of asthma and paternal smoking. When multivariate controls were introduced, the relationships between maternal smoking and asthma persisted. Estimated attributable risks indicate that between 18 per cent and 34 per cent of the asthma reported in these samples can be attributed to maternal smoking. Implications of these findings for primary care physicians are discussed.