Myotonic dystrophy (DM) is the most common adult muscular dystrophy and follows an autosomal dominant pattern of inheritance. Up to now, the clinical diagnosis of DM was based on symptoms presented such as encephalopathy, facies myopathica, paresthesia, atrophy, myotonia, mental retardation, cataract, diabetes, cardiac conduction defects and electromyography. Since 1991 the specific molecular defect in DM is known and a respective diagnosis is possible. The mutation responsible for DM is the expansion of an unstable trinucleotide repeat, (CTG)n, in the 3'-untranslated region of the myotonin protein kinase gene. It is now generally accepted that the CTG repeat length correlates with the clinical category and the age at onset of the disease; therefore genetic tests are essential in monitoring and management of DM-patients and their family members. Based on the average incidence in Europe about 1000 affected individuals can be expected in Austria, a high percentage of whom is, however, not recognized as carries of the DM-mutation. After having established a genetic diagnosis in Austria allowing the detection of this mutation in DM-patients and their relatives, improvement of the diagnostic procedure should be possible.