A 45-year-old female patient was admitted to our hospital presenting with a right anterior cervical tumor that was elastic, hard, painless and 3.5 x 3.5 cm in size. Laboratory data including serum calcium level, thyroid and parathyroid hormonal functions revealed no abnormalities. Further examination, consisting of computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (US) and radioisotope (RI)-imaging demonstrated that it was a solid tumor located behind the right thyroid lobe. The tumor, whose origin was unclear, was successfully removed with right thyroid lobectomy. Macroscopically, it appeared as reddish solid tumor consisting of small cystic lesions storing chocolate-colored mucous. Immunohistology confirmed that there was proliferation of chief cells with positive parathyroid hormone (PTH) staining. Thus the tumor was diagnosed as parathyroid adenoma despite a lack of clinical evidence for hyperparathyroidism. The reason for the lack of clinical features of hyperparathyroidism in this adenoma still remains unclear, however, there might be inactivation of hormone excretion, possibly due to insufficiency of blood supply. This was a very rare case of parathyroid adenoma exhibiting no clinical evidence of hyperparathyroidism.