Hypercalcemia and elevation of a serum PTH level (9800 pg/mL (normal: 160-520) were found in a 72-yr-old woman who had a lung cancer. She underwent pulmonary lobectomy for a suspected PTH-producing lung cancer. However, hypercalcemia and elevation of the serum PTH level were persistent postoperatively. Subsequent examination, using parathyroid scintiscanning, revealed a hot spot in the right lower part of the thyroid gland, suggesting hypercalcemia caused by a parathyroid tumor. She underwent bilateral exploration of the neck; however, four apparently normal parathyroid glands were seen. Therefore, hemithyroidectomy was performed for the possibility of an intrathyroidal parathyroid adenoma. Serum calcium and PTH levels declined after this operation. A nodular lesion was found in the cut sections of the resected specimen, which was consistent with the result of the scintiscanning. Histological examinations revealed a papillary adenocarcinoma of the thyroid gland, and the PTH-immunoreactivity in the tumor cells was confirmed. These findings strongly suggest that PTH could be produced ectopically by the papillary adenocarcinoma of the thyroid gland.