From 1946 through 1982, 30 cases of distant malignant disease that metastasized to the thyroid were diagnosed at the Mayo Clinic. Although metastatic thyroid lesions are not uncommon, they generally do not pose a problem because they are small or are associated with evidence of obvious widespread metastatic disease. The most common sources of such lesions are the kidney, breast, lung, and lymphoid tissue. The thyroid metastatic lesion may appear many years after the identification and treatment of the primary cancer. Because the thyroid lesion may be the only important malignant disease remaining, adequate surgical treatment may prove to be life-prolonging or life-saving.