BACKGROUND: Non-adherence to inhaled corticosteroids (ICSs) is a major risk factor for poor asthma control in children. However, little is known about the effect of adherence to ICS on the incidence of asthma exacerbations. The objective of this stdy was to examine the effect of poor adherence to ICS on the risk of exacerbations in children with asthma. METHODS: In this nested case-control study using data from the Dutch PHARMO Record Linkage System, children aged 5-12 years who had an asthma exacerbation needing oral corticosteroids or hospital admission were matched to patients without exacerbations. Refill adherence was calculated as medication possession ratio from ICS-dispensing records. Data were analyzed using a multivariable multiplicative intensity regression model. RESULTS: A total of 646 children were included, of whom 36 had one or more asthma exacerbations. The medication possession ratio was 67.9% (standard deviation [SD] 30.2%) in children with an exacerbation versus 54.2% (SD 35.6%) in the control group. In children using long-acting beta-agonist, good adherence to ICS was associated with a higher risk of asthma exacerbations: relative risk 4.34 (95% confidence interval: 1.20-15.64). CONCLUSION: In children with persistent asthma needing long-acting beta-agonist, good adherence to ICS was associated with an increased risk of asthma exacerbations. Possible explanations include better motivation for adherence to ICS in children with more severe asthma, and reduced susceptibilityto the consequences of non-adherence to ICS due to overprescription of ICS to children who are in clinical remission. Further study into the background of the complex interaction between asthma and medication adherence is needed.