BACKGROUND Successful parathyroidectomy depends on recognition and excision of all hyperfunctioning parathyroid glands. Because histologic definition is limited, multiglandular disease (MGD) is usually determined grossly by means of estimation of gland size and the experience of the surgeon, resulting in frequency varying from 8% to 33%. Normalization of elevated intraoperative intact parathyroid hormone (iPTH) levels after excision of all hyperfunctioning glands is necessary for postoperative normocalcemia and indicates normal secretion of remaining parathyroids. Abnormal hormone secretion measured during operation has been used to define the extent of excision and the incidence of MGD. METHODS One hundred ten consecutive parathyroidectomy patients with no previous neck surgery or history of multiple endocrine neoplasia had intraoperative iPTH assays performed before and after excision of any suspected abnormal parathyroid gland(s). A drop in iPTH level after gland excision predicted postoperative normal calcium levels. RESULTS All patients except one had normalization of serum calcium levels (average follow-up, 15 months). One hundred five patients had only one hyperfunctioning gland removed, and all have remained normocalcemic. Five (5%) patients had more than one gland involved: four had two or more hyperfunctioning parathyroids and one patient, who had a large parathyroid cyst removed, remained hypercalcemic. CONCLUSIONS By using a biochemical assay, instead of estimated size, to predict which parathyroid glands are hypersecreting, the incidence of MGD in primary hyperparathyroidism was found to be 5%.