Multiple endocrine neoplasia type 1 (MEN1) is a rare genetic disorder, resulting fromMEN1gene abnormalities, which causes tumors mainly in the endocrine glands. We experienced a sporadic case of MEN1 complicated with papillary thyroid carcinoma (PTC) and found a novel missense mutation in the patient’sMEN1gene. Her older sister, who showed no typical symptom of MEN1, had a history of PTC, suggesting the presence of another genetic factor involved in PTC development. This case suggests the importance of an individual’s genetic background in the development of MEN1 complications.