Late hypocalcemia appears associated with thyroidectomy for Graves' disease more frequently than with thyroidectomy for other conditions. Of 62 total thyroidectomies done by a single surgeon, 28 were done for carcinoma, 18 for benign disease (primarily nontoxic nodules with a history of radiation therapy to the head and neck (RT)) and 16 for Graves' disease. Mean calcium concentrations measured two months or more after surgery were 9.38 ± 0.07 (SEM)mg/% for patients with cancer, 8.79 ± 0.31 mg/dl for patients with Graves' disease and 9.38 ± 0.08 mg/dl for patients with other benign diseases. No patient without Graves' disease developed late hypocalcemia. In contrast, six of 16 patients with Graves' developed significant late hypocalcemia requiring calcium therapy. The incidence of hypocalcemia after total thyroidectomy for Graves' disease was significantly greater than that seen in other conditions (p < 0.01). Since no parathyroids were removed in the patients with Graves' disease, and since branches of the inferior thyroid artery were invariably ligated distal to the parathyroids, we hypothesized that the late hypocalcemia might be associated with a peculiarity in scar formation in the presence of this autoimmune disease. Accordingly, parathyroid autotransplantation was performed synchronously as a prophylactic measure in nine subsequent patients undergoing total thyroidectomy for Graves' disease; no instance of late hypocalcemia has occurred in this group. The decreased incidence of late hypocalcemia is highly significant (p < 0.01). Although the precise etiology of late hypocalcemia after thyroidectomy for Graves' disease remains undetermined, this experience indicates that synchronous parathyroid autotransplantation is beneficial in preventing this complication.