Purpose: This study was performed to evaluate the diagnostic usefulness of double-phase Tc-99m MIBI parathyroidism scintigraphy with single photon emission computed tomography (SPECT) in patients with hyperparathyroidism. We also evaluated the relationship between Tc-99m MIBI uptake and oxyphil cell contents in parathyroid glands. Materials and Methods: The subjects were 28 parathyroid glands of 10 patients who underwent Tc-99m MIBI parathyroid scintigraphy and parathyroidectomy for clinically suspected hyperparathyroidism. Early and delayed pinhole images were obtained at 15 minutes and 2 hours after injection of Tc-99m MIBI, and SPECT images were followed. The weight and oxyphil cell contents of parathyroid of tissue were obtained from pathologic specimen, and the scintigraphic findings were compared with histopathology. Results: In surgical histopathology, 6 parathyroid adenomas and 9 parathyroid hyperplasias were confirmed. The sensitivity, specificity, and positive predictive value of early and delayed images were 46.7% (7/15), 76.9% (10/13), 70% (7/10) and 667% (10/15), 92.3% (12/13), 90.9% (10/11), respectively. SPECT image detected an additional small hyperplasia. The sensitivity, specificity, and positive predictive value of combined interpretation of early and delayed images with SPECT were 733% f11/15), 100% (13/13), 100% (11/11). The sensitivity was 100% (6/6) for aenoma, whereas that was 555% (5/9) for hyperplasia. Both adenomas and hyperplasias showed significantly increased oxyphil cell contents compared with normal parathyroid glands (p<0.0001), but the oxyphil cell content and weight were not significantly different between adenomas and hyperplasias. Conclusion: Double- phase Tc-99m MIBI parathyroid scintigraphy with SPECT is useful for lesion localization m patients with hyperparathyroidism. Although both adenoma and hyperplasia have increased oxyphil cell content, the sensitivity is high in adenoma, but low in hyperplasia.