This is a case report of a 63-year-old man with myotonic dystrophy in whom an ECG showed atrial flutter, complete A-V block, idioventricular rhythm with image of left bundle branch block and giant, wide and negative "T-U" waves. The microscopic study of the conduction system showed slight fibrosis of the SAN, fatty infiltration and cellular atrophy of the AVN and bundle of HIS, as wells ad basophilic degeneration of a cell at the begining of the left branch and diffuse sclerosis of the left subendocardial ramifications. Advanced coronary atherosclerosis was also found. Since only two complete pathologic descriptions of the heart in myotonic dystrophy have been reported, no definitive specificity could be atributed to this findings. It is suggested that in the present case both coronary atherosclerosis and an intrinsic myocellular dystrophy could have contributed to the final lesions of the conducting system.