The familial form of medullary thyroid carcinoma (MTC) is caused by mutations of the RET protooncogene. We registered 60 multiple endocrine neoplasia (MEN) 2A patients, 12 familial non-MEN medullary carcinoma (FMTC) patients, and three MEN2B patients with a confirmed RET germline mutation. All 60 MEN2A patients had RET mutations in a cysteine-rich domain. Seven of the FMTC patients had a mutation in cysteine-rich domain, and the other five had a mutation in codon 768, which encodes a tyrosine-kinase domain. Two of the MEN2B patients had a mutation in codon 918, and one patient had a double mutation, one in codon 804 and the other in codon 806, both of which are all encoded tyrosine-kinase domain. The genotype-phenotype correlations of our data will allow individualized recommendations for the optimal timing of prophylactic surgery.