To evaluate the effect of allergic bronchopulmonary aspergillosis (ABPA) on FEV percent predicted in children and adolescents with cystic fibrosis. Longitudinal data analysis (2008-2010). Patients participating in the European Cystic Fibrosis Society Patient Registry. 3350 patients aged 6-17 years. FEV percent predicted was the main outcome measure (one measurement per year per child). To describe the effect of ABPA (main explanatory variable) on FEV while controlling for other prognostic factors, a linear mixed effects regression model was applied. In 2008, the mean (±SD) FEV percent predicted was 78.6 (±20.6) in patients with ABPA (n=346) and 88 (±19.8) in those without ABPA (n=2806). After considering other variables, FEV in subjects with ABPA on entry to the study was 1.47 percentage points lower than FEV in patients of similar age without ABPA (p=0.003). There was no FEV decline associated with ABPA over the subsequent study years as the interaction of ABPA with age was not significant (p>0.05). For patients aged 11.82 years (population mean age), poor body mass index had the greatest impact on FEV in 2008, followed by high-risk genotype (two severe mutations), female gender, diabetes mellitus, chronic infection and ABPA in descending order of effect size. In contrast to the common clinical belief of ABPA having a serious impact on lung function, the difference in FEV between young patients with and without the complication was found to be modest when the effect of other prognostic factors was considered.