Although up to 24% of metastatic cancers have been reported to spread to the thyroid gland, metastases to the thyroid are not detected in clinical practice in most cases. The prognosis is poor when metastatic cancer to the thyroid occurs. The aim of this study was to examine the clinical presentation, cytopathological findings, and clinical course of secondary cancers of the thyroid. The medical records of a total of 1013 histopathologically verified thyroid cancer patients treated during the period from January 1977 to December 1995 in Chang Gung Medical Center in Linkou were analyzed retrospectively. There were 14 patients (1.4% of all thyroid cancers) with secondary cancers of the thyroid with a mean age of 55.3 +/- 16.7 years. All these patients underwent thyroid ultrasonography and a fine-needle aspiration cytology of the thyroid (FNAC) before biopsy or surgical treatment. Tissue diagnosis was obtained by biopsy or necropsy in 12 or by thyroidectomy specimens in 2 patients. Most of the patients died within 9 months of diagnosis except for 2 patients who were lost to follow-up after transferring to another hospital and 1 patient with lymphoma. Before the surgical diagnosis, there were only 7 patients who metastatic neoplasms to the thyroid gland were diagnosed by FNAC. Anaplastic thyroid carcinoma was diagnosed in 5 patients. Benign nodule was diagnosed in 1 patient and lymphoma in another patient. Most of these patients had widespread metastases to many organs, as well as the thyroid gland. As a result these patients had very short survival times. Delayed diagnosis of the thyroid metastasis was the main reason for the short survival period. In conclusion, most of the metastatic lesions presented as an advanced stage of primary cancers. FNAC was a useful tool in the diagnosis. Histopathological diagnosis by surgical open biopsy is needed for the final diagnosis.