We studied 20 patients with myotonic dystrophy (MD) type I, mean age 34.4±12.3 years. A control group consisted of 10 healthy people, mean age 35.2±13.7 years. Cognitive status was assessed using the Brief Cognitive Rating Scale (BCRS), the Frontal Assessment battery (FAB), the Clock drawing test, the Luria memory ten-word retrieval test. The characteristic signs for cognitive deficit in MD were disturbances of visual-spatial functions revealed even in patients with high score on BCRS and FAD, the decrease in verbal fluency, generalization ability and the volume of auditory-speech memory. MRI data indicate the involvement of the gray matter (cortex athrophy) and the white matter (dilatation of the ventricular system, strengthening of the perivascular spaces, areas of T2 and FLAIR hyperintensity) in the pathological process. The lesion of the white matter in MD is similar to the imaging of demyelinization that should be taken into account in making the diagnosis by experts in neuroimaging, neurologists and geneticists.