A case of primary hyperparathyroidism sustained by an unusually large parathyroid adenoma is presented. The tumor affected a 45-year-old woman with a 15-year history of nephrolithiasis and presented as a palpable neck mass. On the basis of clinical findings and ultrasound examination, it was initially misdiagnosed as a thyroid nodule. CT scan and transesophageal endosonography gave a correct definition of the tumor, which was located behind the left thyroid lobe and expanded posterior to the pharynx and the esophagus in the prevertebral space. At surgery a parathyroid tumor measuring 8 x 7 x 3 cm and weighing 90 g was successfully removed. No signs of malignancy were observed by both morphological and cell kinetic analyses.