A patient with primary hyperparathyroidism underwent cervical exploration and hemithyroidectomy. Only one normal parthyroid gland was found and was removed. Hypercalcemia persisted and subsequent arteriography localized a large mediastinal adenoma which was excised. Parathyroid autotransplantation of a small part of this tissue was performed and the patient was well for over a year. He again became markedly hypercalcemic and graft-dependent elevation of parathromone levels was demonstrated. Autograft resection resulted in normocalcemia. Nineteen months later hypercalcemia and elevated parathormone levels prompted re-exploration of the graft site and another enlarged implant was removed. This restored normocalcemia and normal parathromone levels. Parathyroid adenomatous tissue has the potential for autonomous hyperfunction, and caution must be exercised in its use in autotransplantation.