On the basis of the results of histological, histochemical, and electron microscopic studies of pathologically altered parathyroid glands (PTG) in primary and secondary (chronic renal insufficiency) hyperparathyrosis a classification of tumors of this organ is suggested. It takes into account the fact that PTG tumors have a complex histological structure and heterogeneous cellular composition: chief dark, chief clear, transitional oxiphilic, exiphilic, small (round), spindle cells, and C-cells with different structural and functional orientation. Staining with Sudan-III of frozen sections of PTG tissue in primary hyperparathyrosis may be recommended as an additional test for differential diagnosis between adenoma and hyperplasia taking into account that the cytoplasm of chief cells of adenoma contains Sudanophilic bodies, while the cytoplasm of small cells of adenoma and chief (dark and clear) cells of hyperplasid PTG does not. The appearance of C-cells in the thymus, thyroid glands and adenomatous PTG in primary hyperparathyrosis may be considered to be a compensatory-adaptative response to hypercalciemia. Formation of C-cell adenomatosis in a normal PTG combined with PTG adenoma indicates the activity of these glands.