Rationale: The progression of lung changes in cystic fibrosis (CF) from infancy through adolescence remains poorly understood as a result of limited longitudinal imaging data. Objectives: To assess changes in lung morphology and perfusion in children with CF through the pediatric age range by longitudinal chest magnetic resonance imaging (MRI). Methods: 1,112 annual chest MRI scans were performed in 226 patients with CF aged 0–18 years. MRI was assessed using a validated MRI scoring system. Results: The MRI global score continuously increased from 5.5 ± 4.6 at infancy (0 yr) to 17.9 ± 8.5 at adolescence (12–18 yr), and the MRI morphology score increased from 5.0 ± 3.9 to 12.3 ± 6.1 (P < 0.001). Bronchiectasis/wall thickening prevalence increased from 89.1% at infancy to approximately 100% at preschool age (1–5 yr), and the subscore increased from 3.1 ± 1.9 at infancy to 6.6 ± 2.1 at adolescence (P < 0.001). Mucus plugging prevalence increased from 55.4% at infancy to 83.0% at adolescence, and the subscore increased from 1.2 ± 1.6 to 3.7 ± 2.6 in the same period (P < 0.001). Perfusion abnormalities were found in 44.4% at infancy, and increased to approximately 90% at preschool age (P < 0.001). The MRI perfusion score increased from 1.1 ± 1.6 at infancy to 5.6 ± 3.0 at adolescence (P < 0.001). Chronic Pseudomonas aeruginosa infection was associated with higher MRI scores at school age (6–11 yr; P < 0.05–0.001). Conclusions: This is the first study to assess longitudinal changes in lung morphology and perfusion in CF throughout the pediatric age range, providing percentiles as age-specific references for lung disease severity. Our data may facilitate the use of MRI as an endpoint in clinical trials in children with CF. Clinical trial registered with (NCT 00760071 and NCT 02270476).