The effects of residential exposure to environmental tobacco smoke were studied in a sample of 6,529 Canadian children under 15 years of age. Univariate analysis indicated an elevated risk of bronchitis among children whose mothers were single, or under age 25, or whose income or educational level was low. Multivariate analysis indicated that the relative risk of bronchitis among children whose mothers were current smokers relative to those whose mothers never smoked was 3.0 (95% CI:1.6,5.2). Significant dose-response relationships between risk of bronchitis and amount smoked as well as duration of exposure were observed. Using the composite exposure score of all smokers in the household as the source of exposure, the risk of bronchitis decreased slightly but remained significantly elevated. Examination of the risk conferred by the mother's smoking relative to the total household exposure showed that most of the elevation in risk was associated with the mother's smoking contribution. Asthma was also studied, but failed to show a statistically significant association with smoking.