BACKGROUND: MRI with xenon-129 gas (Xe MRI) can assess airflow obstruction and heterogeneity in lung diseases. Specifically, Xe MRI may represent a sensitive modality for future therapeutic trials of cystic fibrosis (CF) therapies. The reproducibility of Xe MRI has not yet been assessed in the context of a multi-site study. PURPOSE: To determine the same-day repeatability and 28-day reproducibility of Xe MRI in children with CF. STUDY TYPE: Four-center prospective, longitudinal. POPULATION: Thirty-eight children (18 females, 47%), median interquartile range (IQR) age 12 (9-14) years old, with mild CF (forced expiratory volume in 1 second (FEV FIELD STRENGTH/SEQUENCE: 3-T, two-dimensional (2D) gradient-echo (GRE) sequence. ASSESSMENT: Xe MRI, FEV STATISTICAL TESTS: Inter-site comparisons were conducted using Kruskal-Wallis nonparametric tests with Dunn's multiple-comparisons tests. Differences for individuals were assessed using Wilcoxon matched-pairs tests. Bland-Altman tests were used to evaluate same-day repeatability, 28-day reproducibility, and inter-reader agreement. A P-value ≤0.05 was considered significant. RESULTS: Median FEV DATA CONCLUSION: Xe MRI may offer excellent same-day and short-term reproducibility. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 2.