BACKGROUND Transient hypocalcemia is one of the postoperative complications of thyroidectomy for Graves' disease, and perioperative parathyroid hormone (PTH) assays are used to predict postoperative hypocalcemia. We evaluated long-term changes in parathyroid function after surgery for Graves' disease. METHODS Serum PTH values were measured in Graves' patients with postoperative hypocalcemia, and those patients were followed postoperatively. RESULTS Subtotal thyroidectomy was performed in 275 patients with Graves' disease. Their serum calcium levels were measured on postoperative day (POD) 1, and patients with transient postoperative hypocalcemia were treated with calcium and vitamin D supplementation and followed up. The amount of calcium and vitamin D supplementation was adjusted to keep the patient's serum calcium level within the normal range. Measurement of their serum intact PTH value on POD 1 revealed normal value in 18 patients, a below normal level in 22, and an above normal level in the other 2. During the follow-up period, the serum iPTH values remained normal in 12 patients, recovered to the normal level in 21 patients, and rose above the normal range in 9 patients. The serum iPTH values of all patients eventually reached the normal range during the follow-up period. A marked difference in preoperative serum alkaline phosphatase concentration was observed between the high-iPTH patients and the normocalcemic patients. CONCLUSIONS The phenomenon of an elevated serum PTH level after surgery for Graves' disease was observed in 21% of the patients with postoperative hypocalcemia despite the achievement of normal serum calcium levels by calcium and vitamin D supplementation.