Myotonic dystrophy, the commonest muscular dystrophy of adult life and the most variable of all muscular dystrophies follows autosomal dominant inheritance and is determined by a genetic locus on chromosome 19. The development of DNA probes on this chromosome, some of them representing specific genes, has provided a series of closely linked markers for myotonic dystrophy, which together with the construction of a series of somatic cell hybrid lines has allowed a detailed localization on the long arm of the chromosome. Existing markers are already able to provide accurate tests of genetic prediction, including prenatal diagnosis. Analysis of the relevant region of 19q by pulsed field gel electrophoresis and other molecular techniques is now in progress to identify the myotonic dystrophy gene itself.