The association of exposure to environmental tobacco smoke (ETS) at home with asthma and several measures of wheeze was examined among 11,534 children aged 8 to 11 yr in 24 communities in the United States and Canada in 1988 through 1990. Information on the child's respiratory symptoms in the past year and history of exposure to ETS was provided by the child's mother on a questionnaire. After adjusting for potential confounders, children currently exposed to ETS were at greater risk of wheezing with colds (odds ratio [OR] = 1.7; 95% confidence interval [95% CI], 1.4 to 1.9), going to a hospital emergency room for wheeze (OR = 1.6; 95% CI, 1.2 to 2.2), and having persistent wheeze (OR = 1.4; 95% CI, 1.1 to 1.8). The relative odds of these symptoms increased with exposure level, and there was no evidence of a difference in the association with smoking by mother, father, or other adults. In contrast to wheeze symptoms, active doctor-diagnosed asthma and asthma medication use were not significantly associated with ETS exposure at home, possibly reflecting underdiagnosis of asthma, reporting bias, or smoking cessation by parents whose child is labeled asthmatic. We conclude that exposure to ETS is associated with wheezing symptoms, medical therapy for wheezing, and wheezing-related emergency department visits in U.S. and Canadian children.