A recent American registry analysis in cystic fibrosis (CF) children showed less lung function decline after starting inhaled corticosteroid (ICS) use. We therefore examined the influence of ICS treatment on lung function in Belgian CF patients. Data from patients ≥ 6 yrs of age were eligible, provided entries on lung function, height and ICS use were available in two consecutive years. Data after oral steroid use or transplant were excluded. 852 subjects contributed data with 2,976 data pairs analysed, 44.9% concerning years of ICS use. Yearly % predicted decline in forced expiratory volume in 1 s (FEV₁) was 1.07% lower during ICS use (p = 0.001). Subgroup analysis for age revealed that the lower FEV₁ decline rate during ICS use was only statistically significant in children 6-12 yrs of age (2.56%; p = 0.0003). Baseline FEV(1) was lower by 5.89% (p < 0.0001) in ICS users for all age groups combined, but there was no difference in baseline lung function in the children 6-12 yrs of age. In 6-12-yr-old children with CF, baseline lung function was similar in ICS users and nonusers, but annualised FEV₁ decline was 2.56% pred lower in ICS users. Our data therefore support recent American findings.