Two hundred twenty-one patients undergoing thyroidectomy were analyzed for factors increasing the risk of postoperative hypocalcemia. Eighty-three percent of all patients experienced hypocalcemia postoperatively, with 13 percent requiring some treatment for symptoms. Patients with advanced thyroid cancer, Graves' disease, or other manifestations of preoperative hyperthyroidism had significantly increased rates of hypocalcemia compared with patients with small cancers or benign euthyroid disease. Total thyroidectomy, repeat thyroidectomy, and thyroidectomy plus neck dissection all significantly increased the incidence of permanent hypocalcemia, whereas lobectomy or subtotal thyroidectomy for benign euthyroid disease were low risk operations. Inadvertent excision of more than one parathyroid gland during thyroidectomy also significantly increased the rate of permanent hypocalcemia.