Medullary thyroid carcinoma (MTC) is a thyroid malignancy originating from C cells. To date, serum calcitonin measurement and fine-needle aspiration biopsy (FNAB) have been prominent diagnostic approaches to these lesions. Although an elevated serum calcitonin level strongly suggests the presence of MTC, this examination cannot identify the origin of the calcitonin overexpression, especially in patients demonstrating multiple thyroid nodules. For the treatment planning, it is important to know which nodule is MTC, especially in nonhereditary MTC. In this study, we propose calcitonin measurement in the washout of FNAB needles after sampling each tumor (FNAB-CT) as a new approach to diagnosis of MTC. We performed FNAB-CT for five MTC cases. Although only one of these cases was definitively diagnosed as having MTC by FNAB cytology, FNAB-CT values ranged from 17,000 to 560,000 pg/mL, which were exceedingly higher than those of 11 controls (seven papillary carcinomas, two adenomatous nodules, one chronic thyroiditis, and one normal thyroid), which showed values ranging from <10 to 67 pg/mL. In patients with MTC, FNAB-CT values were 74 to 1888 times greater than serum calcitonin values. These findings suggest that FNAB-CT can be an additional approach to diagnosis of MTC.