OBJECTIVE To describe our experience with mediastinal parathyroid adenomas diagnosed by endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) and measurement of parathyroid hormone. METHODS We describe the clinical and pathologic findings and diagnostic techniques used in 2 study patients. RESULTS Patient 1 was a 54-year-old man with persistently elevated serum calcium and parathyroid hormone concentrations despite removal of a right inferior parathyroid adenoma. An echoendoscope was used to identify the lesion and to perform FNA. The parathyroid hormone concentration measured in the aspirated material was 1800 pg/mL. Pathologic examination of the resected specimen revealed a 29.7-g parathyroid adenoma. Patient 2 was an 86-year-old woman with recurrent hyperparathyroidism. A linear array echoendoscope was used to perform FNA of the lesion in her mediastinum. The parathyroid hormone concentration measured in the aspirated specimen was 6905 pg/mL. CONCLUSIONS Preoperative localization of recurrent or persistent hyperparathyroidism is often difficult. EUS-FNA allows evaluation of masses, such as those found in the mediastinum, that are poorly evaluated by other imaging modalities. This technique may be a useful adjunct in diagnosing mediastinal parathyroid adenomas.