Myotonic dystrophy (MyD) is a hereditary neuromuscular disorder of an autosomal dominant trait. MyD is caused by an expansion of unstable CTG trinucleotide repeat in the 3′ untranslated region of mRNA coding myotonin protein kinase (MT-PK). We analyzed CTG repeat expansion in 10 patients with congenital MyD and their relatives using the non-radioactive PCR Southern method. The region containing the CTG repeat was amplified by PCR using specific primers. The PCR products were electrophoresed on a 1% agarose gel and transferred to a nylon membrane. The CTG repeat expansion was shown using a fluorescein-labelled (CTG) 10 probe. To estimate the number of CTG repeats, we compared the smears obtained on Southern blotting with a picture of PCR products and a DNA size marker (100 bp). We compared our results of radioactive Southern blotting for genomic DNA digested by Eco RI or Bgl I and for PCR products. In congenital MyD patients, heterogeneous smears (3.89-10.22kb: about 1252-3362 CTG repeat) were observed, whereas in the adult type MyD had heterogeneous smears (0.92-1.82kb: about 262-562 CTG repeat). In asymptomatic MyD patients, there were heterogeneous smears (0.35-1.16kb: about 72-342 CTG repeat). These results demonstrated anticipation. We conclude that the non-radioactive PCR Southern method is useful and convenient for the DNA diagnosis of MyD.