[en] The purpose of this study was to evaluate parathyroid scans obtained by early and late imaging following ^99mTc-MIBI injection. ^99mTc-MIBI scintigraphy was performed 15 min and 2 hr after intravenous injection of 300 MBq (or 370 MBq) of ^99mTc-MIBI. The subjects were 37 patients with primary hyperparathyroidism (adenoma, 34 cases; hyperplasia, 3 cases; and carcinoma 1 case) operated on between January 1994 and March 1996. Sensitivities for localization of adenoma, hyperplasia, and for the both lesions were 84%, 54%, and 76%, respectively. These results were poorer than the results of ultrasonography (US) (adenoma 91%, hyperplasia 54%, and both lesions 81%), and better than by ²⁰¹TlCl/^99mTc-O₄^- subtraction scintigraphy (adenoma 69%, hyperplasia 23%, and both lesions 55%). Parathyroid gland weight and associated thyroid lesions decreased the sensitivity of localization detection using ^99mTc-MIBI. Combined use of ^99mTc-MIBI scintigraphy and US yielded higher sensitivity (89%), than obtained by either method alone. In the 53 cases operated on between April 1996 and March 2000 (adenoma, 47 cases; hyperplasia, 4 cases; carcinoma, 1 case; and diagnosis uncertain, 1 case), the sensitivities of ^99mTc-MIBI scintigraphy for the localization adenoma, hyperplasia, and for the both lesions were 83%, 20%, and 71%, respectively, versus 98%, 30%, and 85%, respectively, for US. It is possible that the light weight of parathyroid glands in the patients with hyperplasia decreased the sensitivity of late imaging. ^99mTc-MIBI scintigraphy is of high diagnostic value in terms of parathyroid function, and US in terms of morphology. The combination of ^99mTc-MIBI scintigraphy and US appeared to be the optimal diagnostic tool for localization of the lesion in hyperparathyroidism. (K.H.)