This paper deals with patients who have both a primary thyroid cancer and a primary malignant tumor of another organ. Fourteen such cases were in the authors' material and 165 cases reported by others are reviewed. The pattern of the tumors associated with thyroid cancer corresponds to the reported frequency of neoplasms of various sites, with the exception that there is an excessive number of cases of leukemia. These may be related to radio‐iodine therapy for the thyroid cancer. If a patient has a cancer and then develops a malignant nodule in the thyroid, this is much more likely to represent a metastasis than a second primary neoplasm; however, it is exceedingly difficult, if not impossible, to differentiate between the 2 on clinical grounds. Usually, metastatic cancer to the thyroid gland is a late manifestation of tumor dissemination but occasionally it may appear early. In such cases, it may be necessary to explore the neck to determine whether the lesion is a second primary or a metastasis.