BACKGROUND: Hypocalcemia is a possible sequela of thyroidectomy; the causes are not fully understood. METHODS: We analyzed 804 patients (594 total thyroidectomy, 209 emithyroidectomy) treated in our Institute from January 1995 to December 2000. Serum calcium, ionized calcium, parathyroid hormone (PTH), fosforemia were screened pre- and postoperatively. RESULTS: Hypocalcemia, defined by a serum calcium less than 7,5 mg/dL, occurred in 126 patients (21,2%). In 90,6% of these patients the serum calcium was normal at seven days after thyroidectomy. In two patients we have registered at 180 days after thyroidectomy a permanent hypoparathyroidism. CONCLUSIONS: Several factors are important in the incidence of postthyroidectomy hypocalcemia but the inadvertent excision of parathyroid gland, ischemia and injury are the major causes of lowering of serum calcium concentration.