We performed ultrastructural studies on nuclear abnormalities in biopsied muscles from seven patients with EDMD, of three non-related families, and two sporadic cases. The diagnosis was based on clinical data and molecular findings. We detected different degrees of abnormalities in the sarcolemmal nuclei ranging from marked condensation of chromatin to complete damage of nuclear components. Other nuclei in the same muscle cell very often appeared normal. The extrusion of nuclear chromatin into sarcoplasm as a consequence of nuclear membrane disintegration was observed in numerous nuclei. All these nuclear changes are considered to be cytological indicators of nuclear dysfunction evoked by emerin deficiency.