Quantitative MR of muscle is increasingly important as potential outcome measure for therapy development in DMD. Since therapy is aimed at preserving or improving muscle tissue, an early marker that reflects muscle state with a suitable dynamic range is essential. Unfortunately, water T2 and %fat do not meet this criteria. Therefore, we aimed to assess whether phosphodiester (PDE)-levels detected by 31P MRS could fill this gap. We have shown a two-fold increase in PDE-levels compared to controls and its detection prior to structural changes which confirm the potential of PDE as an early marker for disease activity in DMD patients.