A 49-year-old female with a metastatic renal cell carcinoma to the thyroid is presented. The thyroglobulin (Tg) level in tumor aspirates of this patient was 37.3 micrograms/l, while that of 49 patients with primary thyroid nodules ranged from 2.0 x 10(3) to 7.5 x 10(8) micrograms/l. Ultrasonogram of the thyroid demonstrated a well-demarcated oval nodule with a cystic region, the solid part of which was hypoechoic and homogeneous. However, similar ultrasonic findings were observed in only 1.1% of 1,054 patients with primary thyroid nodules. Determination of Tg in tumor aspirates and ultrasonography of the thyroid may contribute to the differentiation of metastatic thyroid nodules from primary thyroid nodules.