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Radiation-induced pneumonitis (RP), diagnosed 6–12 weeks after treatment, is a complication of lung tumor radiotherapy. So far, clinical and dosimetric parameters have not been reliable in predicting RP. We propose using non-contrast enhanced magnetic resonance imaging (MRI) based functional parameters acquired over the treatment course for patient stratification for improved follow-up. 23 lung tumor patients received MR-guided hypofractionated stereotactic body radiation therapy at a 0.35 T MR-Linac. Ventilation- and perfusion-maps were generated from 2D-cine MRI-scans acquired after the first and last treatment fraction (Fx) using non-uniform Fourier decomposition. The relative differences in ventilation and perfusion between last and first Fx in three regions (planning target volume (PTV), lung volume receiving more than 20 Gy (V20) excluding PTV, whole tumor-bearing lung excluding PTV) and three dosimetric parameters (mean lung dose, V20, mean dose to the gross tumor volume) were investigated. Univariate receiver operating characteristic curve - area under the curve (ROC–AUC) analysis was performed (endpoint RP grade ≥ 1) using 5000 bootstrapping samples. Differences between RP and non-RP patients were tested for statistical significance with the non-parametric Mann–Whitney U test (α = 0.05). 14/23 patients developed RP of grade ≥ 1 within 3 months. The dosimetric parameters showed no significant differences between RP and non-RP patients. In contrast, the functional parameters, especially the relative ventilation difference in the PTV, achieved a p -value < 0.05 and an AUC value of 0.84. MRI-based functional parameters extracted from 2D-cine MRI-scans were found to be predictive of RP development in lung tumor patients. [Display omitted] • Non-contrast enhanced functional imaging during lung SBRT at low-field MR-Linac. • Non-uniform Fourier decomposition to extract ventilation and perfusion over treatment. • Ventilation changes in high-dose region predictive of radiation-induced pneumonitis. • Higher predictive performance of functional than dosimetric parameters. [ABSTRACT FROM AUTHOR]