Differential access and utilization of medical care by the poor and rich may contribute to differences in asthma prevalence. We therefore studied the relationship of socioeconomic status (SES) to various indicators of asthma in the Canadian context of universal access to medical care. Information on respiratory symptoms, demographics, and home exposures of 1,111 primary school children was collected by questionnaire. Parental occupation was used to establish SES. Exercise-induced bronchospasm (EIB) after a 6-min free-running test was our measure of airways responsiveness and was available for 989 children. As compared with children from the most advantaged homes, children from the least advantaged homes were more likely to present EIB (OR: 2.26, 95% CI: 1.12 to 4.58) and to report night cough (OR: 2.30, 95% CI: 1.04 to 5.06) and cough with mucus (OR: 3.15, 95% CI: 1.06 to 9.33), while there was no significant excess of the report of wheeze or diagnosed asthma. Among factors potentially linked to SES, the presence of a cat at home (OR: 1.63, 95% CI: 1.02 to 2.61) and lower respiratory infection before 2 yr of age were associated with an excess of EIB (OR: 1.71, 95% CI: 1.16 to 2.52). Our results suggest that unidentified environmental factors contribute to the excess asthma morbidity in poor children.