Postoperative hypocalcemia was studied in 40 patients undergoing total thyroidectomy for a malignancy or massive goiter. Parameters evaluated included serum calcium, phosphate and magnesium levels. All patients exhibited a postoperative decline in serum calcium, however, the lowest serum calcium level was not seen until 48 hours after surgery. Serum calcium levels returned to normal in five to six days after surgery in 37 patients. Five patients required calcium supplementation for either symptomatic hypocalcemia or serum calcium levels lower than 7.0 mg/dl. Only three of these five patients were discharged home on oral calcium supplements. In this series, we discovered that the critical period for monitoring of serum calcium was 24 to 96 hours after surgery. If serum calcium replacement was not needed in the first 72 hours after surgery, it would not be needed during the remainder of the patient's hospital course. In addition, we found that serum magnesium levels should also be monitored in the postoperative period and corrected if low.