Water-clear cell hyperplasia (WCCH) of the parathyroid is defined as an absolute increase in parathyroid parenchymal mass, resulting from proliferation of vacuolated cells in multiple parathyroid glands in the absence of a known stimulus. Its incidence, originally being 12.8% in 1930s, fell to 6.8%, 4.9%, and finally to under 1% of cases of primary hyperparathyroidism over the subsequent decades. Now it seems that the diagnosis of WCCH entirely disappeared during last 2 decades. Possible mediastinal location of parathyroid WCCH should always be kept in mind, even when all 4 glands in their normal localization are found, because one cannot exclude possibility of presence of mediastinal supernumerary glands. This lesion should be distinguished from clear cell adenoma of the parathyroid gland, although the fact of existence of multiple adenomas makes this distinction not straightforward. In the mediastinum, WCCH must be differentiated from lobules of brown fat, metastatic pulmonary, prostatic, female genital tract, and mainly renal clear cell neoplasms, clear cell variant of thymic carcinoma, germ cell tumors either primary or metastatic, artifactual clear cell change of large cell lymphoma, and glycogen-rich clear cell rhabdomyosarcoma of the mediastinum.