OBJECTIVE To describe the clinical course of a patient with atypical cystic parathyroid adenoma manifesting as hypercalcemic parathyroid crisis. METHODS We present a case report and review the relevant literature on parathyroid cysts and atypical cystic parathyroid adenomas. RESULTS A 76-year-old woman was evaluated for weakness, weight loss, urinary incontinence, and memory disturbances. She had severe hypercalcemia (calcium level, 18.3 mg/dL) in conjunction with an elevated parathyroid hormone level of 1,472 pg/mL. She received aggressive hydration, pamidronate, and calcitonin. Ultrasonography revealed a large cystic structure (5.8 cm) at the lower pole of the left thyroid lobe. She underwent surgical exploration of the neck, total thyroidectomy for multinodular disease of the thyroid, and subtotal parathyroidectomy. Intraoperatively, 25 mL of fluid was aspirated from the cystic mass, and the parathyroid hormone level in the fluid was 7,400,000 pg/mL. The final pathologic diagnosis was an atypical cystic parathyroid adenoma. CONCLUSIONS Parathyroid cysts are uncommon and should be considered in the differential diagnosis of a neck mass. Although most parathyroid cysts are nonfunctional, 10% to 15% of such cysts are functional and can rarely manifest as acute parathyroid crisis. Atypical cystic parathyroid adenomas are rare and have an unpredictable clinical course. They exhibit some features of carcinoma but lack the indisputable evidence of malignant disease, such as angioinvasion or metastatic involvement. Treatment options for parathyroid cysts include aspiration, injection of sclerosing agents, and surgical excision.