Myotonic dystrophy (MD) is characterized by myotonia, weakness and extramuscular symptoms, including intellectual impairment. We performed magnetic resonance imaging (MRI) of brain and muscle in 25 MD patients: 81% had cerebral atrophy (severe in 36%); 68% had focal white matter lesions, which were large and multiple in 27%. Brain MRI findings correlated with mental impairment; the severity of both correlated with disease duration. Changes in brain and muscle MRI were progressive with time, but independent of each other. Muscle MRI findings were fatty degeneration and loss of bulk. In the calves, the medial gastrocnemius muscles were involved earliest and the posterior tibial muscles relatively spared. In the thighs the vastus muscles were damaged most often and the rectus femoris least. Focal muscle damage was efficiently visualized, sometimes preceding clinical detection. Muscle MRI was less sensitive than conventional methods for early diagnosis, but ideal for follow-up, owing to its non-invasiveness and examiner-independence.