The operative management of patients with hyperparathyroidism is controversial. Higher rates for persistent hypercalcemia and postoperative hypoparathyroidism are seen in multiple-gland hyperplasia and in bilateral neck exploration. Hyperparathyroid patients undergoing unilateral neck exploration with removal of a single parathyroid adenoma have a rapid clearance of parathyroid hormone (PTH) that declines to undetectable levels within hours after successful parathyroid surgery. We have taken advantage of a sensitive immunoradiometric assay (IRMA) for the secreted biologically active, intact PTH-(1-84) molecule and demonstrated a decline of PTH to less than 40% of baseline values 15 minutes after successful parathyroid adenomectomy in 12 patients. Intraoperative measurement of PTH by modification of this IRMA may complement surgical skill and histopathologic information and has the potential for providing guidance regarding the extent of neck exploration necessary for determining surgical care of hyperparathyroidism.