Myotonic dystrophy, or dystrophia myotonica (DM), is the most common inherited muscle disorder in adults. DM is a multisystem disease in which the most disabling feature is muscle wasting that begins in the distal limb and cranial muscles. The genetic basis for DM is an expanded CTG repeat in the DMPK gene on chromosome 19. The size of the expanded repeat, and the severity of the disease, tend to increase in successive generations. The mechanism by which this unusual mutation leads to muscle wasting, myotonia, cataracts, heart block, and neurobehavioral abnormalities has not been clearly defined. Identification of the DM gene has made it easier to delineate other DM-like disorders that are clinically and genetically distinct. The most common of these is proximal myotonic myopathy (PROMM), which is characterized by early involvement of proximal limb muscles. The genetic locus for another DM-like disorder, called DM type 2, was recently mapped to chromosome 3.