Parathyroid cystic adenomas are often misdiagnosed as thyroid cysts and routine preoperative diagnostic tools, such as ultrasonography (US) or 99m technetium-sestamibi (99mTc-MIBI) scans, cannot clearly distinguish between these entities. We present a 67-year-old hypercalcemic woman with a cervical cystic lesion who had negative sestamibi scan results. Her laboratory data indicated primary hyperparathyroidism (serum calcium concentration 14.0 mg/dl, phosphate concentration 2.3 mg/dl, and intact parathyroid hormone (PTH) concentration 239 pg/ml). The cervical US and computed tomography scans revealed a large and vertically long cystic mass (12�11�54 mm). A mass was located from the upper end of the left thyroid lobe to the submandibular region and was not clearly distinguishable from the thyroid. For preoperative definitive diagnosis, we carried out a parathyroid fine-needle aspiration (FNA) and PTH assay (PTH–FNA) of liquid aspirated from the cyst. The intact PTH–FNA concentration was 1.28�106 pg/ml, and the patient was diagnosed with primary hyperparathyroidism due to a cystic mass. She underwent a left upper parathyroidectomy and her serum calcium and intact PTH concentration immediately decreased to normal levels. This report describes the usefulness of PTH–FNA for localizing and differentiating an atypical functional parathyroid lesion from nonfunctional tissue in primary hyperparathyroidism.