One of the more common complications of thyroid surgery is postoperative hypocalcemia, which is potentially serious. Its clinical manifestations range from minimal twitching to life-threatening tetany. Affected patients might require a prolonged hospital stay and supplementation with calcium and vitamin D. In cases of post-thyroidectomy hypocalcemia, it is not always easy to predict which patients will require close monitoring of serum calcium levels. We conducted a study to determine whether early (<24 hr) measurement of serum ionic calcium (SiCa) levels can predict the development of post-thyroidectomy hypocalcemia. We retrospectively analyzed the charts of 150 adults (144 women and 6 men) who had undergone total or partial thyroidectomy, and we identified 42 patients (all women) who had either transient (<1 mo; n = 27) or prolonged (1 to 6 mo; n = 15) temporary hypocalcemia. We found that the patients who turned out to have prolonged hypocalcemia had significantly lower early levels of SiCa than did the patients who later developed only transient hypocalcemia (p = 0.000002). Also, patients with prolonged hypocalcemia had a significantly higher incidence of serious sequelae, including carpopedal spasms and signs of tetany. We conclude that early measurement of SiCa is a reliable predictor of prolonged temporary hypocalcemia following total or partial thyroidectomy.