201Tl/99mTc subtraction scintigraphy (TTS) has been recognized as a useful procedure in the preoperative localization of hyperfunctioning parathyroid glands. Ninety-one patients suspected of hyperparathyroidism (HPT) were studied with TTS for the localization of lesions and 59 patients out of those had a neck exploration and parathyroidectomy which allowed for clinical correlation of pathologic findings with the imaging results. The weight of 30 glands of adenoma ranged 0.100 to 11.700 g (1.840 +/- 0.446 g (mean +/- SE)). The weight of 88 glands of hyperplasia ranged 0.025 to 7.000 g (0.623 +/- 0.100 g). Overall sensitivity and specificity were 47% and 54%, respectively. The sensitivity for the adenoma and the hyperplasia was 87% and 33%, respectively. The specificity for them was 57% and 52%. There was significant difference for the detection between adenomas and hyperplasias. TTS is more useful for the localizing diagnosis of parathyroid adenomas than parathyroid hyperplasia in a patient with hyperfunctioning parathyroid state.