OBJECTIVE: To assess the impact of measured versus reported environmental tobacco smoke (ETS) exposure on asthma severity and exacerbations in an urban paediatric population. DESIGN: We analysed cross-sectional data from the Chicago Initiative to Raise Asthma Health Equity study that followed a cohort of 561 children aged 8-14 with physician-diagnosed asthma between 2003 and 2005. Participant sociodemographic data and asthma symptoms were gathered by parental survey; exposures to ETS were determined by salivary cotinine levels and parent report. Multivariable negative binomial and ordered logistic regressions were used to assess associations between ETS and asthma outcomes. RESULTS: Among 466 children included in our analysis, 58% had moderate or severe persistent asthma; 32% had >2 exacerbations requiring a hospitalisation or an emergency room visit or same day care in the previous year. Half of caregivers reported that at least one household member smoked. In multivariable analyses, salivary cotinine was significantly associated with frequently reported exacerbations in the previous year (adjusted incidence rate ratio=1.39, 95% CI 1.09 to 1.79), but not significantly associated with asthma severity. Reported household smoking was not significantly associated with either asthma severity or frequency of exacerbations. CONCLUSIONS: Salivary cotinine was more predictive of asthma exacerbation frequency but caregiver- reported household smoking was not. Use of a nicotine biomarker may be important in both the clinical and research settings to accurately identify an important risk factor for asthma exacerbations.