Myotonic dystrophies or dystrophia myotonica (DM) is a clinical syndrome that includes myotonic dystrophy type 1 (DM1), myotonic dystrophy type 2 (DM2), myotonic dystrophy type 3 (DM3), and so forth. The terminology was recommended by the new nomenclature for myotonic dystrophies of an International Panel for Consensus. Previous studies have shown that DM1 is caused by the expansion of a cytosine-thymine-guanine (CTG) repeat in the DM protein kinase gene on chromosome 19, and DM2 is caused by an expansion of a cytosine-cytosine-thymine-guanine (CCTG) repeat in the zinc finger protein 9 (ZNF9) gene on chromosome 3. Because DM1 and DM2 have very similar clinical presentations, the diagnosis of these two disorders needs to be confirmed by molecular genetic analysis. Recently, DM3 was reported to include a multisystem myotonic disorder with frontotemporal dementia, and a linkage to chromosome 15q21-24. Although the age at onset, disease severity, and cerebral abnormality on a brain magnetic resonance spectrometry may correlate with the number of triplet repeats in the blood cells of DM1, it is too early to reach a conclusion. In Taiwan, the prevalence of DM1 is much lower than in Western countries. Previous studies have shown that the central nervous system symptomatology is correlated mainly with the white matter lesions in the brain MRI, but the CNS manifestations seem unrelated to the numbers of CTG triplet repeats in the blood cells. The inverse correlation between age at onset and CTG repeat length is significant only in patients with small expansions of about 100-250 triplet repeats. Transmission contraction of the repeat size is likely to occur in alleles with large repeats and is associated with paternal transmission. In congenital DM1, individual variability of muscle differentiation does occur, in spite of the same number of CTG repeats in the leukocytes.