Noninvasive diagnostic procedures for the preoperative localization of parathyroid tumors were performed on a 59-year-old woman with hypercalcemia. In this case, sonography and computed tomographic scanning could not pinpoint the location of the tumor. Thallium-201 and technetium-99m subtraction scintigraphy was also inconclusive in the precise localization of the tumor, but it did define the location retrospectively. However, magnetic resonance imaging (MRI) clearly demonstrated the parathyroid tumor as a high signal on T2-weighted image in the left upper part of the superior mediastinum. The intrathymic parathyroid oxyphil adenoma was resected. Therefore, MRI was considered an effective tool for the localization of parathyroid adenoma, particularly mediastinal parathyroid adenoma.