Familial nonmultiple endocrine neoplasia medullary thyroid cancer accounts for 10% to 15% of hereditary medullary thyroid carcinoma and is characterized by lack of accompanying endocrine or nonendocrine diseases. Simultaneous occurrence of medullary and papillary thyroid carcinoma in the same patient is rare and known as collision tumor. Here, the authors present familial nonmultiple endocrine neoplasia medullary thyroid cancer in 4 sisters, all having RET proto-oncogene polymorphism in exon 15 at codon 904 and 2 having additional polymorphism in exon 13 at codon 769. The index case had concomitant medullary and papillary thyroid carcinomas, which are suggested to be completely different tumors in terms of incidence, cell origin, histopathologic features and prognosis. Histopathologically, she also had Hashimoto thyroiditis in the remaining thyroid tissue and medullary thyroid carcinoma metastasis in 3 cervical lymph nodes. This case is the first in the literature to report coexistent familial nonmultiple endocrine neoplasia medullary thyroid cancer and papillary thyroid carcinoma related with a RET polymorphism (S904S in exon 15).