Environmental tobacco smoke (ETS) exposure has been associated with increased use of the emergency department (ED) for acute asthma care. The purpose of this study was to determine the prevalence of ETS exposure among children presenting to the ED for acute asthma care and whether ETS exposure affects acute asthma severity or response to therapy. We conducted a multi-center study of children 2-11 years with physician-diagnosed acute asthma presenting to 44 EDs in 18 states. Chi-square test, Student's t-test, Wilcoxon rank sum test, and logistic regression were used for the analyses. The study population included 954 children. Thirty-six percent (95% CI, 33-39%) of caregivers reported that their child was exposed to ETS. Among exposed children, 35% were exposed 1-6 days/week, and 65% were exposed daily. Compared to unexposed children, ETS-exposed children were older at asthma diagnosis, older at ED presentation, and were less likely to be Hispanic. Indicators of chronic asthma severity were higher among unexposed children (i.e., total number of medications, use of controller medications, use of beta(2) agonists, number of urgent clinic visits, and lifetime hospitalizations). There was a weak association between ETS and acute asthma severity. Response to therapy (including ED disposition) did not differ between groups. On multivariate analysis, ETS-exposed children were more likely to be older, female, non-Hispanic, have lower household income, not use controller medications, and have a pet at home (all P < 0.05). Our study showed that the prevalence of ETS exposure among children presenting to the ED with acute asthma differs across demographic factors. There were no significant differences in acute asthma symptoms or response to ED therapy between ETS-exposed and unexposed children. Lower use of controller medications and less frequent urgent clinic visits among ETS-exposed children suggest inadequate asthma care or milder disease. The weak association between ETS exposure and acute asthma severity might reflect confounding by psychological factors and/or chronic asthma severity. The frequency of ETS exposure suggests that the ED may be an appropriate venue to engage caregivers of children with asthma in asthma education and smoking cessation efforts.