Hypomagnesemia after total thyroidectomy has not been studied extensively. Our anecdotal experience suggests that it may be important in some patients after thyroid excision. The hypomagnesemic hypocalcemic syndrome has been described in other disease states in which a state of functional hypoparathyroidism exists. This study was designed to determine the incidence of hypomagnesemia after total thyroidectomy and relate it to hypocalcemia and symptoms during the postoperative period. A prospective study of all patients undergoing total thyroidectomy between September 1994 and July 1996 was performed. Patient data, thyroid function, retrosternal extension, initial versus reoperative surgery, operative details, parathyroid resection, and pathology were recorded. Calcium, magnesium, electrolytes, blood count, liver function tests, and albumin were measured prior to surgery and twice daily during the postoperative period. Fifty patients underwent total thyroidectomy: 68% were hypocalcemic, 72% were hypomagnesemic, and 36% were symptomatic during the postoperative period. Hypomagnesemia and gender were associated with hypocalcemia. Volume of fluid and neck dissection were associated with low magnesium levels. Hypomagnesemia and parathyroid resection were risk factors for symptoms after thyroidectomy. No patients developed permanent hypoparathyroidism. Transient hypocalcemia and hypomagnesemia occur frequently after total thyroidectomy. The etiology of this phenomenon is probably multifactorial. Patients are more likely to be symptomatic when both cations are low, and attempting to correct only hypocalcemia may prolong symptoms. It is important to monitor both calcium and magnesium levels after total thyroidectomy and to correct deficiencies to facilitate prompt resolution of symptoms.