Preoperative localization of parathyroid tumors by computed tomography (CT), thallium-201/technetium-99m pertechnetate subtraction scintigraphy (Tl-201/Tc-99m), ultrasonography (US), and magnetic resonance imaging (MRI) was compared in patients with hyperparathyroidism (HPT) to examine the characteristics of each method. A total of 87 patients with HPT were divided into two groups according to the time when they were examined. Patients in group I were examined before MRI had been introduced in our hospital, and a 2.5-MHz transducer probe was used for US. Those in group II were examined by MRI and US using a 7.5-MHz transducer probe. Group I included 45 patients (36 with primary hyperparathyroidism [PHPT] and 9 with secondary hyperparathyroidism [SHPT]), and group II included 42 patients (15 with PHPT and 27 with SHPT). In both PHPT and SHPT and SHPT of group I and PHPT of group II, there was no significant difference in detection rates between all diagnostic methods. In patients with SHPT in group II, the detection rate was significantly higher for CT than for Tl-201/Tc-99m and MRI (both p less than 0.01), and for US than for Tl-201/Tc-99m (p less than 0.01). In both groups I and II, the detection rate of each study method was significantly higher in patients with PHPT than in those with SHPT (all p less than 0.01). Compared with group I, the rate was significantly improved in group II, in both types of patients. Regarding the location of the parathyroid tumor, the detection rate of CT was significantly higher for upper parathyroid glands than for lower glands, whereas that of US and Tl-201/Tc-99m was significantly higher for lower glands. The detection rate sharply increased when the tumor weight reached 250 mg (CT, US) or 1,000 mg (Tl-201/Tc-99m, MRI).