Myotonic muscular dystrophy is the most frequent autosomal muscular dystrophy affecting adults and children. It affects multiple organ systems and is probably the best example of variable expressivity in a human disease. This article presents a patient with congenital myotonic dystrophy who had facial dysmorphism, hypotonia, talipes, feeding and respiratory difficulties in the neonatal period and later presented to us with developmental delay and had percussion myotonia. His mother had clinical and electrophysiological features of myotonia. Expansion of unstable CTG trinucleotide repeat in the myotonic protein kinase gene was demonstrated in both. The identification of this molecular defect allows its specific diagnosis in relation to other neuromuscular disorders as well as accurate prenatal diagnosis.