Saline hypertonic in preschoolers and lung structure as measured by computed tomography.

1. Diagnosis of CF as evidenced by one or more clinical feature consistent with the CF phenotype or positive CF newborn screen AND one or more of the following criteria:

a) A documented sweat chloride ¡Ý 60 mEq/L by quantitative pilocarpine iontophoresis (QPIT)

b) A documented genotype with two disease-causing mutations in the CFTR gene

2. Informed consent by parent or legal guardian

3. Age ¡Ý 36 months and ¡Ü72 months at Screening visit
4. Ability to comply with medication use, study visits and study procedures as judged by the site investigator

5. Ability to execute a technician controlled or spirometer controlled chest CT scan

1. Chest CT within 8 months prior to the Screening visit

2. Acute intercurrent respiratory infection, defined as an increase in cough, wheezing, or respiratory rate with onset within 3 weeks preceding Screening or Enrolment visit

3. Acute wheezing at Screening or Enrollment visit

4. Oxygen saturation < 95% (<90% at centres above 4000 feet elevation) at Screening or Enrollment visit

5. Other major organ dysfunction, excluding pancreatic dysfunction

6. Physical findings that would compromise the safety of the participant or the quality of the study data as determined by site investigator

7. Investigational drug use within 30 days prior to Screening or Enrolment visit

8. Treatment with inhaled hypertonic saline at any concentration within 30 days prior to Screening or Enrolment visit

9. Start of any additional inhaled saline solution at any concentration, or other hydrating agent such as mannitol or mucolytic drug such as dornase alpha within 30 days prior or following the Screening or Enrollment visit

10. Chronic lung disease not related to CF

11. Inability to tolerate first dose of study treatment at the Enrolment visit