Background: Intraoperative parathyroid hormone (IoPTH) testing is useful in the management of hyperparathyroidism. The successful removal of hypersecreting parathyroids is indicated by a decrease in PTH levels >50% within 15 min. A subset of patients with mild hyperparathyroidism will actually have starting PTH levels in the normal range. We sought to determine if IoPTH testing is necessary in these patients and if the 50% rule delineating surgical cure is reliable. Methods: A retrospective review was performed on all patients who underwent parathyroidectomy for hyperparathyroidism at a single institution from 3/2001 to 8/2008. Results: Of the 1,001 patients, 142 (14%) had mild hyperparathyroidism and normal baseline PTH levels (<65 pg/ml). Their mean PTH was 59 ± 1 pg/ml. During surgery, 105 (74%) had a >50% decline in PTH levels after resection of hyperfunctioning parathyroid glands, and their operations were terminated. In contrast, 37 (26%) patients did not have a >50% decline in PTH levels leading to further surgical exploration. In these 37 patients, the PTH levels fell by >50% after the removal of the additional glands in 25 patients (17.6%) and dropped after 20 min in 7 patients (4.9%). In 5 patients (3.5%) the IoPTH did not drop. Of the 142 total patients, 91 had single adenomas and 51 patients had multi-gland disease. All patients (100%) were cured (normal serum calcium after 6 months). Conclusions: Intraoperative PTH testing plays an important role in the operative management in 14% of patients with mild hyperparathyroidism. Importantly, a 50% decline in IoPTH level within 15 min of parathyroidectomy is 96.5% reliable in predicting cure in these patients with PTH starting in the normal range. © 2011 Société Internationale de Chirurgie.