Mediastinal exploration by sternotomy was carried out 36 times in 35 patients with evidence of hyperparathyroidism amongst a group of 1,461 operations for hyperparathyroidism between 1962 and 1987. The exploration was indicated 30 times as re-operation for hyperparathyroidism persistent after one or more negative cervicotomy procedures or recurrent, and 6 times from the outset, (5 in a context of acute hypercalcemia). Exploration was positive 20 times (16 adenomas, 2 hyperplasias, 2 metastases of a parathyroid carcinoma), but negative 16 times (3 diagnostic errors, 6 cervical lesions discovered subsequently, 7 explorations totally negative). The site of mediastinal parathyroid lesions is usually intrathymic (13 cases) but may sometimes (3 cases) be in the middle mediastinum. Pre-operative investigation to identify the site of a possible mediastinal lesion, in re-operation surgery, remains unreliable, the best investigation in this series being thoracic CT scan with 63% true positives. Thallium-Technetium subtraction isotope scan, carried out once with a good result, should now have a place amongst investigations. The principal post-operative complication is hypoparathyroidism which can be prevented by cryopreservation of a fragment of the lesion for possible subsequent reimplantation. Mediastinal exploration is rarely indicated in hyperparathyroid surgery and should be undertaken only after meticulous cervical exploration.