A 59-year-old female with diabetes mellitus presented with hypercalcemia and polycythemia. Her serum calcium and intact parathyroid hormone (iPTH) levels were increased, and Tc-99m sesta-MIBI scanning showed hot uptake in the lower portion of the left thyroid lobe. After parathyroidectomy, her calcium, iPTH, and polycythemia were normalized. In conclusion, the differential diagnosis of polycythemia and hypercalcemia should also include the possibility of a parathyroid tumor in addition to other neoplasms.