The imaging methods currently used to localize the parathyroid gland in patients with hyperparathyroidism have been criticized as unreliable and even misleading. We evaluated a new imaging technique that uses technetium Tc 99m sestamibi and iodine 123. We studied 21 patients having a diagnosis of primary hyperparathyroidism and no previous parathyroid surgery. Scintigraphy to localize the abnormal parathyroid was done before operation. A solitary adenoma was localized in 14 patients. Six patients had images consistent with diffuse hyperplasia, and one patient had a dual adenoma. The surgical and histologic findings confirmed the preoperative data. The sensitivity of this method is 87.5%, the specificity is 100%, and the predictive value is 100%. This new method of preoperative localization of abnormal parathyroid glands is useful in patients having initial neck exploration for primary hyperparathyroidism.