Functional parathyroid cysts (FPCs) and nonfunctional parathyroid cysts (NPCs) are 2 distinct clinical and histologic entities. Review of prospective clinical database records. Tertiary academic center. Patients enrolled in a prospective surgical database between January 1, 1990, and May 31, 2007. Cervical exploration for primary hyperparathyroidism or cervical mass. Age, sex, morbidity, imaging results, pathologic findings, cyst characteristics (size, location, and fluid), and perioperative calcium and parathyroid hormone levels. Cystic parathyroid lesions were found in 48 of 1769 patients (3%) studied. Functional parathyroid cysts were more common than NPCs, arising in 41 of 48 patients (85%), and showed no predisposition for sex or embryologic origin. Single-photon emission computed tomographic imaging failed to localize FPCs in 12 of 37 patients (32%). The fluid in FPCs was clear or colorless in 9 of 15 characterized specimens (60%). Rupture of cystic parathyroid lesions during resection was associated with prolonged elevation of intraoperative parathyroid hormone levels (P =.045). Cystic parathyroid lesions weighing 4 g or more were associated with the development of postoperative symptomatic hypocalcemia (P =.03). Functional parathyroid cysts occurred exclusively in adenomas with cystic change, whereas NPCs (with 1 exception) were without associated adenoma on final histologic examination. Cystic parathyroid lesions often contain turbid or colored fluid, and FPCs are more common than NPCs. Neck cysts of uncertain origin should be diagnostically aspirated for parathyroid hormone content. During resection, cyst rupture should be avoided, and patients with large cysts should be managed expectantly to forestall the development of symptomatic hypocalcemia. Functional parathyroid cysts and NPCs are likely 2 separate clinical and histologic entities.