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Facioscapulohumeral dystrophy (FSHD) is a debilitating neuromuscular disorder characterized by progressive muscle weakness, leading to significant morbidity. Despite its impact, approved therapies for FSHD are lacking, with only a handful of compounds in development. The urgent need for effective treatments underscores the importance of identifying biomarkers that correlate with disease activity or progression to facilitate clinical management and evaluate potential therapeutics for FSHD. Since inflammation plays a central role in FSHD pathophysiology, we are particularly interested in inflammatory biomarkers and have previously identified Interleukin-6 (IL-6) as a promising severity biomarker. To validate this hypothesis, we conducted two independent longitudinal studies: the CTRN-FSHD France (NCT04038138) and the Cytokine FSHD (NCT04694456), each enrolling 30 adult ambulant FSHD1 patients ((N=60, 53.2 ± 15.05 years, 53% men, age at onset 32.8 ± 16.1, D4Z4 repeat number 6.6 ± 0.2). Comprehensive clinical assessments, IL-6 measurements, and whole-body muscle Magnetic Resonance Imaging (MRI) scans were performed at baseline and after 12 months. We systematically investigated the association between IL-6 concentration and clinical as well as MRI data. Our results demonstrate a robust correlation between IL-6 levels and established clinical severity metrics (including Clinical Severity Score, Manual Muscle Testing sum score, 6-minute Walk Test, and Domain 1 of the Motor Function Measure-32 items) both at baseline and at the 12-month follow-up reaffirming IL-6 as a serum biomarker of FSHD severity. Furthermore, we observed a significant association between IL-6 levels and muscle composition, with higher IL-6 concentrations correlating positively with increased muscle fat infiltration and edema. This suggests a potential role for IL-6 in modulating muscle composition, possibly through its pro-inflammatory effects. Importantly, the longitudinal analysis revealed that changes in IL-6 levels over time positively correlate with changes in MRI composite scores, indicating a dynamic relationship between IL-6-mediated inflammation and the observed alterations in tissue structure or pathology detected by MRI. In conclusion, our findings not only confirm IL-6 as a reliable serum biomarker of FSHD severity but also highlight its close association with muscle composition changes. The observed correlation between IL-6 levels and MRI findings underscores the potential of IL-6 as a valuable tool for monitoring disease progression and evaluating treatment response in FSHD patients. [ABSTRACT FROM AUTHOR]