We followed 37 patients with myotonic dystrophy for a mean of 6 years. Two developed atrial flutter or fibrillation, 6 developed a new bundle branch block, 1 developed complete heart block requiring a pacemaker, and another with progressive 1st-degree heart block and a widening QRS interval had a sudden death. Most patients had predictable, gradually progressive disease of their cardiac conduction system. We recommend that patients with progressive atrioventricular block or widening QRS interval due to myotonic heart disease have yearly ECGs and be questioned about syncope or presyncope to determine the need for a cardiac pacemaker.