Most cases of primary hyperparathyroidism (80 to 90%) are caused by a parathyroid adenoma; most of the rest are caused by either parathyroid gland hyperplasia or multiple adenomas. Parathyroid carcinoma can be the cause of primary hyperparathyroidism in 1 to 5% of patients. When the appropriate clinical scenario is presented, it must be considered in the differential diagnosis. We describe the case of a 72-year-old woman with a long history of hypercalcemia and arthritic shoulder and neck pain who was admitted with an elevated calcium level and acute renal failure secondary to bilateral obstructing ureteral calculi. A sestamibi scan and magnetic resonance imaging detected the presence of what appeared to be a right inferior parathyroid adenoma. Her intact parathyroid hormone (PTH) level was 2,257 pg/ml. Following placement of bilateral ureteral stents and adequate hydration, the patient was taken to the operating room for a neck exploration and removal of a parathyroid adenoma with rapid intraoperative PTH monitoring. A 4 x 2.5-cm, 10-g mass was removed from the right inferior pole of the thyroid gland. Following excision, her intraoperative PTH level dropped from 1,103 to 110 pg/ml. Her ionized calcium levels fell from a high of 8.4 mg/dl preoperatively to 4.7 mg/dl. On final pathologic examination, the mass was confirmed to be a parathyroid carcinoma.