Myotonic dystrophy (m.d.) is an autosomal dominant multisystem disorder involving muscles, brain, heart, eye, endocrine system, alimentary and respiratory systems. M.d. is the most frequent cause of muscle dystrophy. Unstable CTG trinucleotide repeat at 3' untranslated end of the myotonic protein kinase gene on chromosome 19q 13.3 is the molecular basis of the disease. Normal length of CTG trinucleotide repeat is 5-40. Molecular mechanism of the myotonic dystrophy is discussed. Cataract, heart dysfunction, endocrine organs dysfunction, gallbladder stones, impotence are characteristic changes in patient with m.d. Apathy, drowsiness, sometimes dementia point to central nervous system involvement. Clinical course, correlation between CTG expansion and clinical manifestation are described. Nowadays progress in molecular genetic allows to make the diagnosis by DNA examination. Prenatal diagnosis is also possible.