A 59-year-old woman with primary hyperparathyroidism was found to have a parathyroid adenoma behind the left clavicle. Preoperatively, it appeared as a hypoechoic mass on ultrasonography, as a hot nodule on thallium scintigraphy, and as a high signal on T2-weighted magnetic resonance imaging. Histological, immunohistochemical and ultrastructural studies of the surgically resected tumor revealed a parathyroid adenoma composed mainly of oxyphil cells with production of a parathyroid hormone. Moreover, a multilocular lesion of lymphangiectasia was contained. Hypercalcemia was alleviated postoperatively. These observations corroborated a functioning parathyroid oxyphil cell adenoma. This is the first case report of functioning oxyphil cell adenoma of the parathyroid gland with lymphangiectasia in Japan.