Although rare (10% of thyroid cancers), medullary thyroid cancer is remarkable by the presence of a specific biological marker; elevation of blood calcitonin. It allows its preoperative diagnosis and this extensive surgery which is the only efficient treatment. The possibility of hereditary form (30%) must always be kept in mind; they can now be detected by genetic screening since specific mutations on Ret gene have been recently discovered. Follow up and early treatment of at risk subjects in a family is therefore possible and permits definitive surgical cure.