The electron microscopic features of the striated skeletal muscle, the striated cardiac muscle, and the smooth muscle from a woman who had been suffering for many years from myotonia dystrophica with cardiac involvement are described. The skeletal muscle was studied at two different stages of the disease. In the first material the main changes consisted of centrally situated nuclei, disorganisation of the sarcomeres, and focal disruption of the Z-line. The satellite cells were well represented. Three years later atrophy and degenerative, necrotic changes of the skeletal muscle were evident. The satellite cells were absent. Few changes were seen in the striated cardiac muscle. These consisted of slight interstitial fibrosis and large accumulations of mitochondria with intramitochondrial dense granules. The smooth muscle cells of the oesophagus showed disorientated filaments and mild degenerative changes. It is concluded that the skeletal muscle was more severely affected than the other types of muscle.