Cardiac conduction abnormalities are frequent in myotonic dystrophy and can result in complete heart block. In a study of 26 patients with myotonic dystrophy, first degree heart block was found to precede clinical presentation of the disease in 7 patients. His bundle electrocardiography in 11 patients demonstrated abnormalities throughout the entire conducting system, even in patients with only first degree heart block. In patients with block of the right bundle branch or anterior fascicle of the left bundle branch, slowing in the remaining fascicles was always present. Therapy of myotonia with procainamide significantly lengthened the P-R interval, and treatment with diphenylhydantoin significantly shortened the P-R interval suggesting that diphenylhydantoin may be preferable for therapy of myotonia in patients with myotonic dystrophy.