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Parathyroid carcinoma is a very rare disease occurring in less than 2-3% of all the cases showing clinical features of primary hyperparathyroidism. Several histological markers have been used for distinguishing between benign and malignant tumors of the parathyroid glands. However, most of these markers are not easily applicable and clinical prognosis cannot be predicted by histopathological criteria alone. A recent study has drawn attention to the role of the cell cycle associated antigen Ki-67 detected by MIB-1 monoclonal immunocytochemistry in parathyroid tumors: in fact, Ki-67 seems to be a valuable marker of malignancy in such tumors since it permits an easy detection of proliferating and dividing cells. Here we report in detail a case of severe recurrent hyperparathyroidism in a 51-year-old female patient undergoing regular hemodialysis treatment. In the surgical specimens of the parathyroid glands, the tumor proliferative fraction of 56, expressed as the number of Ki-67-positive nuclei per thousand cells, and the mean mitosis count of 0.5, expressed as the percentage of the total amount of Ki-67 positive nuclei, support the diagnosis of parathyroid carcinoma despite the scanty amount of microscopical signs considered characteristic of malignancy, i.e. extensive thick fibrous bands or prominent nucleoli. To our knowledge this paper is the first clinical report that supports the diagnostic role of the cell cycle associated antigen Ki-67 in parathyroid carcinoma in a case of secondary hyperparathyroidism in a patient undergoing hemodialysis.