In 7 patients with metastatic trophoblastic disease, there was laboratory evidence of increased thyroid function. Clinical hyperthyroidism was either not present or minimal, and thyroid gland size was normal. There was uniform elevation of 24-hr radioiodine uptake, serum PBI and BMR, and the serum cholesterol was depressed. This syndrome of altered thyroid function in association with metastatic trophoblastic disease had 3 distinctive features: 1) In spite of laboratory findings of hyperthyroidism, clinical evidence of thyrotoxicosis was usually minimal; 2) in those patients whose tumors responded to treatment with Methotrexate the thyroid function tests returned to normal; 3) evidence of increased hormone secretion was present, whereas normal or decreased thyroidal function has usually been seen with malignancies. Plasma TSH levels were determined in 2 patients and found to be elevated; assay of the tumor from 2 patients revealed TSH activity greater than that of equivalent amounts of blood. It is postulated that these alterations in thyroid function may stem from the production by the tumor tissue of a substance with TSH activity and that this represents another example of the varied humoral syndromes associated with a great variety of neoplasms.