Many children treated with cystic fibrosis transmembrane conductance regulator (CFTR) modulators increase their body mass index (BMI). To extend this observation, we prospectively monitored rates of change for growth, body mass and composition indices and z scores, forearm strength, and FEV1 during ETI therapy for a cohort of 27 children, ages 6-11, assessed potential risk factors for concerning change rates; and documented variability of individual change rates. Body composition was assessed by bioelectrical impedance analysis (BIA). Z score calculations used matched data from US children. Outcomes rates of change were analyzed using a linear mixed effects regression model. At enrollment, median BMI z-score was 0.6, percent body fat was 22.7, and percent predicted (pp) FEV1 was 100. During ETI treatment, mean z scores for annualized change rates of BMI (0.02±0.07), fat mass index (FMI) (0.02±0.08), and fat free mass index (-0.03±0.07) were not different from zero. Percent increase of FMI (mean:12.0±29.0) often exceeded that for skeletal muscle mass index (SMMI)(4.7±4.9). Older children and girls experienced the largest weight gain. Individual body mass and composition index rates of change varied extensively. FEV1 increase was independent of body composition and muscle strength change rates. Preadolescent children, as a group, experienced growth and body composition trajectories expected of US children, adding muscle mass, but preferentially adding fat mass when treated with ETI. Individual outcomes variation suggests potential benefit for body composition monitoring and interventions to promote healthy physical maturation during ETI therapy.