Background Lung delivery of plasmid DNA encoding the CFTR gene complexed with a cationic liposome is a potential treatment option for patients with cystic fi brosis. We aimed to assess the effi cacy of non-viral CFTR gene therapy in patients with cystic fi brosis. Methods We did this randomised, double-blind, placebo-controlled, phase 2b trial in two cystic fi brosis centres with patients recruited from 18 sites in the UK. P atients (aged ≥12 years) with a forced expiratory volume in 1 s (FEV 1 ) of 50–90% predicted and any combination of CFTR mutations, were randomly assigned, via a computer-based randomisation system, to receive 5 mL of either nebulised pGM169/GL67A gene–liposome complex or 0·9% saline (placebo) every 28 days (plus or minus 5 days) for 1 year. Randomisation was stratifi ed by % predicted FEV 1 (