Myotonic dystrophy is an inherited multi‐system disease. Its pathophysiology leading to muscle malfunction and damage is not well understood. ²³Na NMR spectroscopy was applied here for an in vivo comparative study of the calf muscles of 7 myotonic dystrophy patients at various stages of the disease and 11 healthy volunteers. Both the total sodium content, expressed as the ratio of the ²³Na and ¹H water signals, and the fast transverse relaxation time, T₂₁, determined from the triple quantum‐filtered spectra, increased in correlation with the severity of the disease. The results demonstrate that ²³Na NMR enables the quantitation of myotonic dystrophy progression.