10% of patients with medullary thyroid carcinoma have a family history of this cancer. The genetic nature is much more marked in the associations known by the name of multiple endocrine neoplasms (MEN) type II. Type IIa is compatible with autosomal dominant inheritance. The clinical symptoms may be revealed later, but the response of calcitonin to pentagastrin should allow the diagnosis of all carriers of the gene before the age of 35 years. The incomplete nature of clinical penetrance (probably 80%) may lead to errors of omission in the screening. The disease appears to be less aggressive in certain lines, without any explanation at the present time. The locus of MEN is probably on chromosome 10, in the centromere region or nearby on the long arm. The mechanism of tumourigenesis has not yet been elucidated. Hopefully, in the near future, reliable markers for the identification of children at risk will be available together with an understanding of the genetic heterogeneity of the disease.