PURPOSE: The aims of this study were to determine the incidence and evaluate the risk factors for hypocalcemia after total thyroidectomy and to investigate how many parathyroid glands should be preserved to prevent postoperative hypocalcemia. METHODS: From March 2007 to February 2011, a retrospective review of 866 patients who underwent total thyroidectomy and node dissection for thyroid cancer was performed. The incidence and predisposing factors for hypocalcemia were analyzed. Among them, a total of 191 cases had four of their parathyroid glands identified intraoperatively. These patients were then divided into one preserved parathyroid gland group (group I, n = 22) and two or more preserved parathyroid glands group (group II, n = 169). The incidence of hypocalcemia with regards to the number of preserved parathyroid glands was determined and the results between the two groups were compared. The total calcium, ionized calcium and parathyroid hormone levels were compared between the two groups. RESULTS: The overall incidence of transient and permanent hypocalcemia was 9.2% and 0.5%, respectively. The decreased number of preserved parathyroid gland and increased number of removed central lymph node were the significant risk factors for developing postoperative hypocalcemia. In 191 cases identified with four parathyroid glands, the incidence of hypocalcemia was related to the number of preserved glands (group I, 22.7%; group II, 3.0%; P < 0.001). CONCLUSION: The insufficient number of preserved parathyroid glands is the only cause of hypocalcemia after total thyroidectomy and node dissection. At least one preserved parathyroid gland may prevent postoperative permanent hypocalcemia.