Preoperative localization of the parathyroids using 99m Tc-sestamibi scanning has not yet been established as a routine diagnostic procedure for primary hyperparathyroidism. Several studies have demonstrated a variable degree of accuracy (70%-98%) in asymptomatic patients. Methods: We evaluated the accuracy of this technique in 64 patients who underwent scanning between January 2000 and January 2005 according to the clinical manifestations of the disease. Results: The study included 25 asymptomatic patients (group I), 18 nephrolithiasis patients without overt bone disease (group II), and 21 patients with severe bone involvement and osteitis fibrosa cystica (group III). Mean serum calcium in groups I, II, and III was 10.98 6 0.02, 11.32 6 0.17, and 13.35 6 0.35 mg/dL, respectively. Mean serum parathyroid hormone in groups I, II, and III was 135.45 6 13.50, 165.85 6 15.06, and 579.6 6 628.4 pg/mL, respectively. The 99m Tc-sestamibi scan results were positive in 64% of the patients in group I, in 83% of those in group II, and in 100% of those in group III. Of the patients with severe bone disease, 70% showed increased uptake on the initial images, whereas in the other groups, increased uptake was seen only on the delayed images, as expected. Conclusion: Our data show a high degree of accuracy for the use of 99m Tc-sestamibi scanning as a localizing procedure in severe primary hyperparathyroidism.