OBJECTIVE The aim of this study was to evaluate whether magnetic resonance imaging (MRI) can be used as a noninvasive approach to assessment of disease severity and muscle damage in Myotonic Dystrophy type 1 (DM1). METHODS The MRI findings in legs of 41 patients with DM1 were evaluated with respect to the tibialis anterior (TA) skeletal muscle impairment. Magnetic resonance imaging findings were compared with TA strength measurements obtained by quantitative manual testing, duration of the disease and with the length of the CTG repeats. RESULTS Muscle MRI abnormalities were observed in 80% of DM1 patients, ranging from edema-like abnormalities alone to severe atrophy/fatty replacement. Edema-like abnormalities seem to be an earlier MRI marker of the disease. Fatty infiltration/atrophy correlated with the TA muscle force (r = 0.95), the severity (P = 0.00001) of the disease but not with the duration of the disease (P = 0.3) or the length of the CTG repeats (P > 0.10), measured in peripheral leukocytes. Evaluation of other muscles of the legs revealed that the medial gastrocnemius and soleus muscles were the most frequently and severely affected muscles, while tibialis posterior muscles were relatively spared. Edema-like abnormalities are most frequently observed in the skeletal muscles of the anterior compartment. CONCLUSIONS Muscle MRI is helpful to depict muscle abnormalities but does not seem to be a reliable indicator of skeletal muscle involvement in DM1 since the decrease in TAmuscle force is not correlated with MRI abnormalities in some patients.