To assess the relationship between passive smoking and asthma, we investigated (1) whether passive smoking was more prevalent among asthmatic than control children and (2) whether exposure to tobacco smoke was higher in acute asthma than in nonacute asthma. Three groups were recruited into a case-control study: 72 acute asthmatic children from the emergency room (ER), 35 nonacute asthmatic children from the asthma clinic, and 121 control children from the ER. Both questionnaire and urinary cotinine/creatinine ratio (CCR) were used to assess passive smoking. Levels of CCR greater than or equal to 30 ng/mg were used to identify children exposed at home. Mean CCR was also computed. Acute and nonacute asthmatic children had similar prevalences of passive smoking at home. Acute cases showed a higher mean CCR than nonacute cases, but this was not significant. In comparing all asthmatic to control children, smoking by the maternal caregiver was more prevalent among asthmatic children (odds ratio OR = 2.0, 95% CI 1.1, 3.4). This was confirmed by CCR greater than or equal to 30 ng/mg (OR = 1.9, 95% CI 1.04, 3.35) and by the difference in mean CCR (43.6 versus 25.8 ng/mg, p = 0.06). We conclude that smoking by the maternal caregiver is associated with clinically significant asthma in children. We could not show that it is a trigger of acute asthma attacks.