Postoperative hypocalcemia is a common and most often transient event after extensive thyroid surgery. It may reveal iatrogenic injury to the parathyroid glands and permanent hypoparathyroidism. We prospectively evaluated the incidence of hypocalcemia and permanent hypoparathyroidism following total or subtotal thyroidectomy in 1071 consecutive patients operated during 1990–1991. We then determined in a cross‐sectional study which early clinical and biochemical characteristics of patients experiencing postoperative hypocalcemia correlated with the long‐term outcome. Postoperative calcemia under 2 mmol/l was observed in 58 patients (5.4%). In 40 patients hypocalcemia was considered severe (confirmed for more than 2 days, symptomatic or both). At 1 year after surgery five patients (0.5%) had persistent hypocalcemia. We found that patients carried a high risk for permanent hypoparathyroidism if fewer than three parathyroid glands were preserved in situ during surgery or the early serum parathyroid hormone level was ≤ 12 pg/ml, the delayed serum calcium levels ≤ 8 mg/dl, or the delayed serum phosphorus level ≥ 4 mg/dl under oral calcium therapy. When one or more of these criteria are present, long‐term follow‐up should be enforced to check for chronic hypocalcemia and to avoid its severe complications by appropriate supplement therapy.