The aim of the present study was to assess whether or not there is any correlation between magnetic resonance imaging (MRI) abnormalities and excessive daytime sleepiness (EDS) in a consecutive series of patients with myotonic dystrophy (MD). The influences of nocturnal breathing abnormalities and sleep morphology on EDS were also evaluated. Ten MD patients were studied by means of an all-night polysomnographic recording, the multiple sleep latency test (MSLT) and MRI. Diagnosis of MD was established on the basis of the clinical and electrophysiological evidence of myotonia as well as of the characteristic genetic pattern. No patient had respiratory failure. Polysomnography and MSLT were also evaluated in ten healthy age-matched controls under the same environmental conditions. The mean MSLT value was significantly lower in patients than in controls. Five of the ten patients were found to have pathological EDS. The quantitative sleep variables and the nocturnal apnoeas in these five patients were not significantly different from those of the patients without EDS. As two patients did not undergo MRI because of claustrophobia, the MRI data were considered in eight patients. Corpus callosum (CC) atrophy was detected in four patients, whereas three patients showed hyperintense areas in the white matter. No correlation was found between EDS and MRI indexes of subcortical atrophy as well as volume of the hyperintense areas. By contrast, a correlation was found between the MSLT value and the reduction in the anterior area of the CC. Our data suggest that CC atrophy might occur in MD patients, and that the size of the CC anterior area might be associated with EDS.