To determine the practical usefulness, the findings of ultrasonography (US) and that of computed tomography (CT) were compared with the surgical findings in 8 cases of hyperparathyroidism. US criteria for hyperfunctioning parathyroid gland are the following three findings: (cf. Fig. 1) (1) The lesion is a hypoechoic or cystic mass lesion. (2) The lesion is located at the dorsal side of the thyroid. (3) The lesion has a capsule together with the thyroid. In diagnosing hyperfunctioning parathyroid glands, US sensitivity was 62%, US specificity 100%, and US accuracy 84%. On the other hand, CT sensitivity was 46%, CT specificity 89%, and CT accuracy 72%. Diagnostic rate of US was superior to that of CT. Factors influencing preoperative localization of parathyroid tumors are weight, shape, and site. US could point out parathyroid tumor which weighed not less than 294 mg. Spherical or elliptical parathyroid tumor were easily detected. US could not reveal mediastinal parathyroid tumor.