Backgrounds : Myotonic dystrophy is an autosomal dominant disorder which has two genetically distinct forms. Type 1 is resulted from mutated DMPK (dystrophica myotonica protein kinase) gene which has abnormally expanded CTG repeats in the 3-prime untranslated region on chromosome 19q13. In this study, we aimed to verify the existing concept that severity of clinical symptoms is correlated with length of CTG repeats. Methods : We selected 30 patients with myotonic dystrophy whose mutated DMPK gene had been revealed. And 13 patients are excluded whose initial or follow-up data were insufficient to identify clinical significance. Then we devided 17 patients into two subgroups according to different two criteria. First criteria was clinical phenotype (congenital versus non-congenital) and the other was length of CTG repeats (more than 1,000 versus less than 1,000). Results : In clinical subgrouping, the patients with congenital phenotype whose first symptom was shown within neonatal period were seem to have statistical significance for many variables(congenital hypotonia, dysphagia, prematurity, rate of admission to neonatal intensive care unit, need for respiratory support at birth, future dependency on ventilator and dependency of daily living) compare to non-congenital subgroup. Meanwhile, in genetic subgrouping, only one variable (poor feeding in neonate) had statistical significance in patients whose length of CTG repeat is more than 1,000 compared to the other subgroup. Conculsions : There is no statistical significance between length of CTG repeats and age of onset. It could be due to a small group of patients, but possibility that there is no correlation in reality cannot be ruled out. Clinical variables which are statistically significant should be focused to anticipate their prognosis and prepare the proper management rather than genotype.