We studied the feasibility of employing the measurement of thyroglobulin (Tg) in the washout of the needle used to perform the fine needle aspiration cytology (FNA-Tg) for the differential diagnosis of nonthyroidal neck masses of unknown etiology. We studied 35 patients presenting for 1 or more neck lumps outside the thyroid gland. A previous history of treated differentiated thyroid cancer (DTC) was given by 23 patients and of nonthyroidal malignancy by 3 patients. FNA-Tg was measured in the Tg-free serum used to wash out the needle employed for the cytology. Finally, all patients were treated by surgery. FNA-Tg was always detectable in 14 patients with thyroid cancer metastases demonstrated by histology, with a mean (+/- SD) of 27,087 +/- 37,622 ng/FNA (P less than 0.002) compared to patients without thyroid cancer metastases (mean +/- SD, 12.1 +/- 4.8 ng/FNA in 7 cases; undetectable in 14 cases). Assuming 21.7 ng/FNA (the mean +/- 2 SD of the negative patients) as the cut-off value, all patients with metastases from DTC were detected by FNA-Tg. FNA-Tg had better negative predictive value than cytology, since this last technique gave 10 inconclusive results, comprising 2 false negative results in patients with metastases from DTC. Our results indicate that elevated concentrations of FNA-Tg in nonthyroidal neck nodes strongly suggest the diagnosis of metastases from DTC.