We report the case of a 33-year-old woman who was operated on with the diagnosis of primary hyperparathyroidism (PHP) in 1986. She had bone disease and slight hypercalcemia. Two parathyroid glands were removed with a lack of clinical improvement. Subsequently, the serum calcium levels were normal with occasional slight increases. Depressed Phosphorus values and elevated alkaline phosphatases and PTH levels were also present, associated with severe bone involvement and muscular weakness. A second cervical exploration performed in 1989 disclosed only a normal parathyroid gland, which was not removed. In 1990, a thoracic CT scan showed the presence of a 1 cm mediastinal nodule close to the great vessels. A thoracotomy was performed to remove this nodule, which proved to be a parathyroid adenoma. After surgery, the patient presented with a "hungry bone" syndrome, characterized by very low levels of calcium, phosphorus and magnesium, which required enteral and parenteral calcium and magnesium supplements, plus dihydroxyvitamin D. The association of normocalcemia and intermittent hypercalcemia with severe bone disease is very rare, as is the presence of a mediastinal adenoma. This could explain the difficulty in the diagnosis in this case.