Objective: The objective of the study was to investigate the changes of histopathological and immunohistochemical parameters of breast tissue among the HRT users. Design: Prospective, pilot study. Setting: Departments of Gynecology and Obstetrics, Pathology and Radiology of the 2nd Medical School, Charles University and the Teaching Hospital Motol, Praha 5, Czech Republic. Methods: The samples of breast issue were obtained by core cut biopsy and during the breast surgery procedures in the study group of HRT users (no=48) and in the control group of women without HRT (n=22). Proliferation index (Ki-67), expression of ERB2 oncoprotein and hormonal receptors (ER, PR) were examined in the breast tissue in addition to routine histopathological examinations. Results: We did not record increasing freqeuncy of proliferative and precancerous lesions in the group of HRT users. Ki-67 expression was very low both in HRT users and in the control group of women. The values of estrogen receptors expression in breast tissue samples of women with HRT were similar to the findings in the normal breast. The values of progesterone receptor expression were higher among the HRT users then non-users, but it can be considered as a normal response of breast tissue to hormonal influence. Expression of ERB protein in HRT users was similar to that found in non-users. Conclusion: The findings indicate that HRT has not increased the proliferation rate of the breast tissue in our study group, so this mechanism certainly does not increase the risk of breast cancer.

        Key words: HRT, Ki-67, breast tissue proliferation, breast cancer risk