OBJECTIVE: To evaluate changes in cardiac magnetic resonance (CMR) tissue characteristics in patients with active cardiac sarcoidosis (CS) confirmed by positron emission tomography (PET)-CT undergoing immunomodulatory therapy (IMT), and to explore their potential use for inflammation monitoring. DESIGN: Retrospective observational cohort study. SETTING: Tertiary care referral centre in Germany. PARTICIPANTS: From a cohort of 47 patients with CS, 24 patients with PET-confirmed active myocardial inflammation and complete baseline and follow-up CMR imaging after ≥6 months of IMT were included. PRIMARY AND SECONDARY ENDPOINTS: Primary outcome: Changes in CMR-derived tissue characteristics (T1, T2 mapping, late gadolinium enhancement (LGE) mass). SECONDARY OUTCOMES: Changes in functional (ejection fraction (EF) and global longitudinal strain (GLS)) and morphological parameters (end-diastolic/systolic volume indices (EDVi/ESVi)). RESULTS: Patients with PET-confirmed active CS show increased global T1 and T2 compared with healthy volunteers. Over the course of IMT, significant reductions in global T2 (median (IQR): 39 (38-41) ms vs 37 (36-39) ms; p=0.002), LGE-region T2 (43 (40-46) ms vs 41 (38-42) ms; p=0.003), and relative LGE mass (23% (17-38) vs 15% (8-32); p=0.006) were observed. No significant differences were found in EF (p=0.78), GLS (p=0.49), EDVi (p=0.56), ESVi (p=0.28) or native T1 values (p=0.23). CONCLUSION: In patients with PET-confirmed active CS undergoing IMT, serial CMR demonstrated measurable changes in T2 mapping and LGE parameters, suggesting a potential role for CMR tissue characterisation in monitoring myocardial inflammation. However, due to the observational design and absence of a control group, causal treatment effects cannot be confirmed. Further prospective studies are needed to validate the utility of CMR for treatment monitoring in CS.