Myotonic Dystrophy (MyD) is the most common form of muscular dystrophy affecting adults. Recently a heritable unstable DNA sequence containing CTG repeat in the region associated with MyD was detected. We have analyzed DNA from the members of an MyD family using Southern hybridization and PCR method. Unaffected individuals of this family had no expansion of unstable DNA sequence after EcoRI and BglI digestion and had CTG repeats under 12, while affected individuals had more than 0.2 kb expansion of DNA sequence and had over 50 CTG repeats. We confirmed that the length of unstable MyD region correlated with severity of the disease in a family and that the diagnosis of MyD had been improved by development of DNA probes for the CTG repeats.