The myotonic muscle disorders represent a heterogeneous group of clinically similar diseases sharing the feature of myotonia: delayed relaxation of muscle after voluntary contraction (action myotonia) or mechanical stimulation (percussion myotonia). In classic myotonia the myotonia improves as muscles warm up, whereas in paradoxical myotonia (paramyotonia) it worsens with repeated muscle contractions. Electrophysiologically, myotonia is characterized by the repetitive electrical activity of muscle fibers. Two decades of work on the electrophysiology of myotonia focused attention on the muscle cell membrane as the site of the molecular defects of these diseases. Genetic-linkage studies have now pinpointed the lesions to chromosomal loci . . .