OBJECTIVE: The highest incidence of asthma occurs in children. Proper management requires treatment with multiple expensive medications. For private drug plan subscribers, deductibles and co-payments constitute a user fee which may impede access. The primary objective was to examine the impact of co-pay level on asthma medication use in asthmatic children in private drug plans. METHODS: A cohort of 17,046 asthmatic Ontario children from an aggregated private sector claims database were classified as zero (no co-payment), low (<20%) or high (>20%) co-payment. Use of bronchodilators (BD), inhaled corticosteroids (ICS), leukotriene antagonists (LA), oral corticosteroids (OS) and combinations were examined in 2003. Multiple linear and logistic regressions compared medication use between groups controlling for age and sex. RESULTS: Annual asthma medication claims per child were significantly lower in the high co-pay group (6.6) compared to the zero (7.0) and low co-pay (7.2) groups (p<0.0001). Children in the high co-pay group were less likely to receive concomitant BDs, ICSs and LAs compared to the low co-pay group, Odds Ratio 0.78 (95% CI 0.67, 0.86). As a marker for asthma exacerbation, children in the high co-pay group were more likely to receive an OS compared to the zero co-pay group, Odds Ratio 1.1 (95% CI 1.0, 1.2). CONCLUSIONS: Cost-sharing level affected asthma medication utilization, with the highest cost-sharing group exhibiting significantly lower use of maintenance medications and higher use of medications for acute exacerbation than other groups. These results are valuable to inform decisions regarding policy reform and drug plan management.