The purpose of this study was to document the incidence, nature and source of neoplasms metastatic to the thyroid gland, which were diagnosed by fine needle aspiration (FNA) cytology. In the seven year period from 1986 to 1992, 21 cases were identified with metastatic malignancies in FNA specimens from the thyroid. This represented 7.5% of neoplastic thyroid lesions aspirated in this unit. All patients presented clinically with thyromegaly or discrete nodules. Only five patients were known to have malignancies of other sites prior to FNA. The majority of metastatic nodules were bronchogenic in origin (nine). The gastrointestinal tract (five) and melanomas (two) were the next most frequent sources in the series. Single cases arose in the prostate, larynx, kidney (all carcinomas), and uterus (a leiomyosarcoma). One patient had a thyroid deposit of acute myeloblastic leukemia. This large study demonstrated that tumors of many histological types may involve the thyroid gland, and furthermore, may masquerade as primary thyroid malignancies. Recognition of an alien cell type not only prevents inappropriate thyroid surgery, but may also direct the search for the unsuspected or unknown primary. Metastases to the thyroid gland occur more frequently than is generally appreciated. FNA is the procedure of choice for evaluation of thyroid nodules in general, and thyroid metastases in particular.