The purpose of this study was to directly compare the efficacy of Tc-99m MIBI with TI-201/Tc-99m pertechnetate scintigraphy for localizing parathyroid adenomas in patients with primary hyperparathyroidism. Of 21 patients suspected of having primary hyperparathyroidism who had undergone both studies, 14 had subsequent surgery and pathologic correlation. Based on the surgical and pathologic results, Tc-99m MIBI yielded 13 true-positive and 1 false-negative result. Thallium-201/ Tc-99m pertechnetate imaging yielded eight true-positive, four false-negative, and two nondiagnostic studies because of lack of Tc-99m pertechnetate uptake from thyroid hormone suppression. In conclusion, Tc-99m MIBI scintigraphy is an accurate method for localizing parathyroid adenomas in patients with primary hyperparathyroidism and the results suggest that it is superior to using TI-201/Tc-99m pertechnetate.