FDG-PET plays a potentially important role in detecting various types of cancer. However, in patients with suspected cancer with unknown cause of elevation of serum tumor markers, its role has not yet been defined. We present a patient without previous history of malignant disease, who had progressive elevation of carcinoembryonic antigen (CEA). The conventional diagnostic procedures detected no abnormal findings that might have induced the elevation of CEA. FDG-PET images demonstrated two FDG-avid lesions in both lobes of the thyroid gland. The pathologic findings confirmed the diagnosis of medullary thyroid cancer. The serum CEA level returned to normal range after operation.