The object of this work was to compare the frequency of three polymorphic changes in the RET proto-oncogene: L769L, S836S, and S904S in patients with medullary thyroid carcinoma (MTC; n = 246) and in the general population (n = 420 for single-nucleotide polymorphism [SNP] L769L and S904S; n = 411 for SNP 836). We tried to investigate how the harbored SNPs affect the age at onset of sporadic medullary thyroid carcinoma (sMTC) and MTC in carriers of known pathogenic mutations at codons 634 and 791 of the RET gene. A statistically significant difference was found in the frequency of the heterozygous change L769L in patients with sMTC (48.3%) and in unaffected individuals (39.5%). The presence of the polymorphic change L769L in the RET gene predisposes to the development of sMTC and also lowers the age of onset of MTC in carriers of the homozygous polymorphic variant L769L. The presence of this polymorphic change in MTC patients carrying, at the same time, the RET codon 634 mutation lowers the age of onset of MTC in this group.