For frequent complications of cardiac involvement, it is important to recognize exactly myotonic dystrophy from a cardiovascular standpoint. Electrophysiological and histological studies were performed in three patients. Analysis of the site of the conduction disturbances was revealed by His bundle electrography proximal to the His bundle in one patient and distal in two other patients. The histological examination for biopsied myocardium by light and electron microscopy disclosed replacement of large areas by fat and also showed varying degrees of cellular damage. The most striking abnormality of the heart muscles was found in the mitochondria in which a peculiar and granular substance bulged into the outer space. Both the conduction system and the myocardium were involved in our cases. The value of His bundle recordings and endomyocardial biopsy in evaluating cardiac abnormality in the disease were discussed.