Side localization of parathyroid adenomas was performed by venous sampling for intact parathyroid hormone (PTH) in 20 consecutive patients with primary hyperparathyroidism (pHPT) after induction of anesthesia. The results were thus available during surgery. Nineteen of the patients had solitary parathyroid adenoma, and one had hyperplasia. There was no complication to the procedure. A lateralizing PTH gradient for a parathyroid adenoma was obtained in 13 patients. At surgery 12 of them (92%) were proved correct; that is, the adenoma was localized on the same side. Thus the technique correctly lateralized the adenoma in 12 of 19 patients (63%). We therefore conclude that the method of intraoperative venous sampling for intact PTH is safe, and the predictive value of a lateralizing gradient is high. It could therefore be used as an adjunct to surgical skill and noninvasive localization procedures in selected cases, for instance in patients with prior neck surgery and hypercalcemic crisis.