The goal of this observational study is to provide optimal monitoring and support when initiating ETI treatment in eligible persons with cystic fibrosis (aged 12 y +) and to document on a daily basis, from 72 hours before the start of treatment and then for 14 days i) i) FEV1 changes (home spirometry), ii) ii) respiratory symptoms changes, iii) any possible side effects. Through a dedicated electronic platform, these data will be monitored every day by the medical team, which will be fully available for any questions or concerns patients may have.

Inclusion Criteria:

• Cystic fibrosis diagnosis
• At least 12 years old
• Carrier of at least one copy of F508del mutation
• Ability to perform reliable and reproducible spirometry
• Medical and psychological stability
• written consent
• owning a smartphone

Exclusion Criteria:

• Lung transplant
• FEV1 > 120 % pr (%GLI) at the inclusion test
• Pulmonary exacerbation

CF patients aged 12 y or more, carrying at least one copy of F508del mutation, in stable condition, reliably performing spirometry