Multiple endocrine neoplasia type 2 (MEN 2) is an autosomal dominant cancer syndrome, which is divided into three subtypes: MEN 2A, MEN 2B and familial medullary thyroid cancer (FMTC). Approximately 92% of MEN 2 cases are caused by mutations in exons 10, 11, 13-16 of the RET proto-oncogene. There exists inter- and intra-familial phenotypic variability among the MEN 2 families, even when the disease is caused by the same RET mutation, suggesting a role for genetic modifiers, such as polymorphisms/haplotypes. We have sought to determine the frequency and position of RET germline mutations in a cohort of 114 Spanish probands with any sign of MEN 2, and to search for putative modifier loci. Mutational screening of RET revealed 9 different mutations, present in 26 of the 114 probands (22.8%). In addition, distributions of 8 RET polymorphisms and the haplotypes comprising them, were studied in the context of the families positive for RET mutational screening, in order to evaluate them as genetic modifiers. The relationship between RET mutation type and presence of a polymorphism/haplotype was analyzed. The relationship between the presence of pheochromocytoma (PC) and/or hiperparathyroidism (HPT) in carriers of the same RET mutation, and the genotype for the specific variants was also studied. The results derived from those analyses revealed no associations of any variant/haplotype to a specific mutation or to the clinical presentation. Nevertheless, these observations do not permit us to exclude the possible role of other variants in RET or other related genes, in the final presentation of the disease.