Cystic fibrosis patients with active lung disease will undergo a high resolution PET-CT.PET/CT scans (GE ST Discovery PET/CT scanner) will be performed 60 to 90 minutes after injection of 5 MBq/kg of FDG. PET/CT will be repeated at the end of the treatment and compared with the results of the initial scan. PET and CT will be interpreted by a certified nuclear medicine physician and by a certified radiologist, respectively, blinded to clinical data, using the PET severity score (PSS), based on the number and the intensity of FDG uptake of lung foci. Intensity of uptake will be determined by calculating the mean value for the maximum standardized uptake values (MSUV) of all foci.