Although the benign counterpart of medullary carcinoma of the thyroid has never been indicated in textbooks, we propose that C cell adenoma is a rare but distinct clinical entity. Two patients, a 43-year-old man and a 53-year-old woman, had similar thyroid tumors, both about 4 cm in diameter. The cut surfaces of the resected tumors were indistinguishable from a common microfollicular adenoma of the thyroid. Microscopically, the tumors were uniformly composed of fusiform cells without any follicle formation. Neither amyloid deposition nor calcification was found. Although some kind of C cell tumors were suggested, the exact nature was debatable. However, extremely high levels of calcitonin (1330 and 2065 pg/ml, respectively; normal level, less than 170 pg/ml) were found in the stored sera taken preoperatively. Serum levels of carcinoembryonic antigen (CEA) were normal in both patients. Immunohistochemically, the tumor tissues were positive for calcitonin and neuron-specific enolase but negative for CEA with a monoclonal anti-CEA antibody. No somatostatin, glucagon, or adrenocorticotropic hormone activity was found. It is highly probable that such tumors have not been closely studied and have been regarded as eccentric adenomas of the thyroid or simply as the so-called medullary carcinomas of the thyroid.