Background: About 1-5% of all anterior cystic neck masses account for parathyroid cysts, which can be functional and nonfunctional. Functional cyst or cystic parathyroid adenoma is a very rare tumor which causes primary hyperparathyroidism and relatively hypercalcemia. Here, we present a giant cystic parathyroid adenoma which is the largest reported to date in the literature. Case Presentation: Our patient presented with a right-sided neck mass and manifesting symptoms, which developed recently, such as itching, fatigue, proximal muscle weakness, and polyuria. In this case, the high calcium level had not been taken into account when evaluating the patient; the possibility of parathyroid adenoma had not been suspected; and repeated aspirations had been performed. After parathyroidectomy, the patient developed postoperative hyperparathyroidism as a complication. Conclusion: Even though cystic parathyroid adenoma is a rare diagnosis, it should be considered in differential diagnosis, and parathyroid hormone of the aspirated fluid from the neck mass should be measured.

Keywords: primary hyperparathyroidism, giant cystic parathyroid adenoma, persistent hyperparathyroidism, case report, neck mass, surgery