This is a multicenter, randomized, double-blind, controlled, parallel group trial assessing structural lung disease in children with CF ages 3 to 5 at enrollment. Participants will be randomized 1:1 to receive 7% hypertonic saline (treatment arm) vs. 0.9% isotonic saline (control arm) administered twice daily via jet nebulizer for 48 weeks. Study visits will occur at screening, enrollment, and at Weeks 12, 24, 36, and 48. Parents or the legal guardian will be contacted at Weeks 1, 4 and 8 to document changes in health status, adverse events, concomitant medications/treatments, and encourage study treatment compliance. Parents or the legal guardian will also be contacted approximately every 6 weeks between visit 3, 4, 5, and 6 to address individual issues or concerns related to study treatment or study participation, and to document changes in health status, medications and treatments.