Medullary thyroid cancer (MTC) is genetically determined in 30% to 35% of cases, notably through multiple mutations in the RET protooncogene located on chromosome 10, for which a genotype-phenotype relationship determines age of onset. There are three phenotypes: MEN 2 A and B, and isolated familial MTC. The type of mutation determines 3 levels of aggressiveness. Current guidelines recommend thyroidectomy during the first months of life for patients with very-high-risk (level 3) mutations and before 5 years of age for high-risk (level 2) mutations. There are no precise recommendations for lower-risk mutations, for which the surgical decision also depends on the calcitonin level and family history. We describe 18 patients who underwent prophylactic surgery. Regardless of the mutation, all patients with a normal preoperative calcitonin level were cured. However, surgery was performed later than recommended, for various reasons, including late genetic diagnosis and parents' opposition.