An approximate correlation has been demonstrated between the degree of CTG repeat expansion and clinical severity among myotonic dystrophy patients. Congenital myotonic dystrophy, which is the most severe form of the disease, has the largest size of CTG repeat. Muscle immaturity is a characteristic finding in congenital myotonic dystrophy muscle. We compared the CTG repeat size and histologic findings of skeletal muscle from patients with congenital myotonic dystrophy. An 8.6 kb or 9.8 kb plus an expanding band ranging from 15 kb to 17.5 kb was observed in muscle from five patients with congenital myotonic dystrophy by Southern blot analysis using EcoRI-digested DNAs probed with p5B1.4. There was no correlation between immaturity of skeletal muscle and the degree of CTG repeat expansion on skeletal muscle. Undetermined maternal factors may have an important role in the cause of immaturity of muscle in congenital myotonic dystrophy patients.