Background: Data on lung function in patients with PCD are few and contradictory. Within the EU project BESTCILIA, we compared lung function (FEV₁) of patients with PCD to the Global lung function initiative (GLI) 2012 reference values and to published data from UK patients with Cystic Fibrosis (CF; Goss et al. Thorax 2015).Methods: We calculated z-scores and %predicted values for FEV₁ using the GLI reference, and used a multilevel linear regression model, to account for repeated measurements, and adjusted for age, sex and study centre.Results: We obtained 4683 FEV₁ measurements of 648 PCD patients from 15 centres. The median age was 16 years (range 6-70), 295 (46%) were females. In the multilevel model FEV₁ z-scores (95% CI) were significantly lower than GLI values (FEV₁ -1.5 (-1.6 to -1.4); p<0.005)). Both sexes and all age groups were affected, with a smaller difference for children aged <10 years (FEV₁ z score -1.18 (-1.3 to -1.05). Compared to published data for CF, FEV₁ %predicted (95% CI) was similar in children (e.g. age 6-9: PCD: 87% (86 - 89%); CF: 90% (88 - 91%)); but better in adult PCD patients (age 18-21: PCD: 76% (73 - 80%); CF: 66% (65 - 68%)).Conclusions: This is the largest study ever conducted on spirometry in patients with PCD. FEV₁ was significantly reduced compared to normal reference values for all age groups and both sexes. Young children with PCD and CF both had a low FEV₁, which later in life remained relatively stable in PCD unlike in CF. In the ongoing study, we will analyse determinants of lung function, particularly age at diagnosis, time since diagnosis, and diagnostic certainty.Funding: FP7 grant 305404, SNF32003B-144068.