Aim: Determining the immunohistochemical characteristic of parathyroid glands (PG) proliferative activity in patients with primary and secondary hyperparathyroidism (HPT) using cell cycle and proliferation immunohistochemical markers, Ki -67 and PCNA. Introduction: A few studies results have shown A few studies results have shown significant detection of Ki-67 in hyperplasia due to secondary hyperparathyroidism (sHPT), whereas it s demonstrated only in adenomas in primary HPT (pHPT). The highest PCNA expression is detected in hyperplastic PG in sHPT and in adenoma in pHTP, but in pHTP hyperplasia it s extremely low. Material and Methods: We analyzed the surgically removed PG of 96 patients with HPT. In POSTER SESIJA 64 MATERIA MEDICA • Vol. 34 • Issue 1, suplement 1 • april 2018. addition to standard histopathological parameters the results of immunohistochemical reaction of Ki-67 and PCNA in 23 normal, 73 hyperplastic PG and 23 adenoma were analyzed. Results: 41 (42.7%) patients had pHPT, and 55 (57.3%) sHPT. Within pHPT adenoma was diagnosed in 23 (56.1%) patients and hyperplasia in 18 (43.9%). Detection of PCNA was 94.4% in pHPT hyperplasia, 91% in sHPT hyperplasia, and 83% in adenoma. 22 (98%) of the normal PG didn t have PCNA expression. The expression of Ki-67 was found in 13 (56.5%) adenomas and in 11 (18.3%) nodular hyperplasia. The high statistical significance for Ki-67 (p <0.0001) was found between adenoma and pHPT and sHPT. Conclusion: The results of our analysis showed high Ki-67 and PCNA expression in parathyroid adenomas. Increased Ki-67 expression corresponds with increased cellular proliferation and contributes to tumorigenesis in many organs, but doesn t distinguish accurately benign from malignant PG tumors.