A thyroid tumor about 4 cm in diameter was removed from a 53-year-old female. The clinical features were those of common thyroid adenomas, such as soft, smooth-surfaced and round contour of the tumor, no calcification and no lymph node metastasis. However, microscopically, the tumor was composed of compact cell nests rather similar to paraganglioma without any follicle formation. Histochemical examination revealed its C-cell nature, such as argyrophilia and calcitonin activity, although no amyloid deposit was demonstrated. Extremely high levels of calcitonin were found in the stored serum taken pre-operatively, but serum carcinoembryonic antigen levels were within normal limits. The calcitonin level dropped to the normal range soon after the operation, indicating the absence of residual tumorous tissues. Thus, the tumor behaves as an adenoma with a C-cell nature. Interestingly enough, the benign counterpart of medullary carcinoma of the thyroid seems to be quite rare in humans, and no similar tumors have ever been reported. Variations of C-cell tumors will be discussed in relation to their production and secretion of carcinoembryonic antigen.