Two family are described affected by a mild myotonic syndrome, characterized by almost exclusively presence of myotonic phenomenon, in absence of symptoms as muscular weakness, hypotrophy or hypertrophy ("myotonia levior"). The nosographic classification of this disease in the congenital myotonias is today uncertain, and it could be a case of a low expressivity variant of the dominant form (Thomsen disease), or the heterozygotic phenotype of the recessive form (Becker disease). Family history, clinical and electrophysiological data obtained in the patients seem to support first hypothesis, providing an useful element for a correct genetic counselling in these cases.