OBJECTIVE To evaluate the clinical and pathologic impact of metastases to the thyroid gland. METHODS We studied the clinical/autopsy records and pathologic features of 79 Chinese patients (38 men, 41 women) with secondary tumors of the thyroid over a 26-year period. RESULTS The incidence of thyroid secondaries at autopsy was 0.5% and at thyroidectomies for malignancy was 1.2%. Eighteen (23%) of the 79 patients presented with an enlarged mass in the thyroid. The diagnosis of metastasis in the thyroid gland was suspected clinically in six cases. Fine-needle aspiration cytology detected an 80% malignancy rate, but in only half of such cases was the diagnosis specific for secondary malignancy. Many of the metastatic lesions in the thyroid gland occurred shortly after the detection of primaries (mean latency period, 9 months). The time from diagnosis of the primary tumor to metastasis to the thyroid gland was greater than or equal to 3 years in only eight patients. The majority (81%) of the secondary tumors were carcinomas. Of these, the lung was the most common primary tumor site (43%), followed by the breast (9%) and stomach (8%). Rare sources of primary tumors, such as nasopharyngeal carcinoma, choriocarcinoma, malignant phylloides, and osteosarcoma, were also found. The metastatic lesions in the thyroid gland could be identified macroscopically in 43 patients (54%). Lesions were often solitary (n = 24) rather than diffuse (n = 8) or multiple (n = 11). Microscopic examination revealed that secondary thyroid tumors were often moderate or poorly differentiated adenocarcinomas (n = 42). All the patients died shortly after the appearance of metastatic lesions in the thyroid gland. CONCLUSIONS Metastasis in the thyroid gland is uncommon and may be a diagnostic problem. Its presence often indicates a poor prognosis.