Medullary thyroid carcinoma (MTC) management requires determination of the sporadic or familial nature of the disease. RET proto-oncogene mutation analysis in the tumor tissue obtained at surgery and in the peripheral blood identifies somatic vs. germinal mutations. We now report a case of MTC in which a RET somatic mutation at codon 918 was detected in fine-needle aspiration specimens obtained from both the thyroid nodule and two enlarged neck lymph nodes but not in peripheral blood. Therefore, a diagnosis of sporadic MTC was made before surgery. Thus, this approach, by excluding preoperatively multiple endocrine neoplasia disease, permitted immediate thyroidectomy without search for pheochromocytoma. PCR-based genetic analysis in fine-needle aspiration biopsy specimens, therefore, preoperatively identifies genetic abnormalities at an early and easily manageable stage and may well contribute to the management strategy of MTC.