A disease named for its principal manifestations in skeletal muscle: myotonia-the sustained contraction of muscle in response to electrical or percussive stimuli-and dystrophy-the progressive loss of skeletal muscle with fibrosis and fatty infiltration. It is inherited as an autosomal dominant trait with extremely variable penetrance. The most constant feature is the extreme variation of clinical expression between multiple members of each pedigree. The apparent worsening of the disease in subsequent generations, or anticipation, is characteristic. The occurrence of a congenital form of myotonic dystrophy, inherited by either sex but only through their mother, is another manifestation of the disease. With the discovery of the inherited variable trinucleotide repeat, some of these genetic phenomena are being explained, but the mysteries of myotonic dystrophy are still unfolding.