Sex steroids are known to affect immune responses; however, information on immunomodulatory effects of estrogen/progesterone hormone replacement therapy (HRT) is still limited. Therefore, the present study aimed to investigate the effect of estrogen/medroxyprogesterone HRT on natural killer (NK) cell cytotoxicity and immunoregulatory cytokine (IL-2, IL-4 and IFN-gamma) release by phytohemaglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC) from 15 selected healthy postmenopausal women. NK cell cytotoxicity, cytokine production and serum levels of 17beta-estradiol (E2), progesterone (P) and FSH were tested in each patient before and after 90-days HRT. NK cell cytotoxicity was tested by (51)Cr-release assay using K562 erythroleukemic cells as target. Specific cytokine production and serum hormone levels were evaluated by enzyme-linked immunosorbent assay (ELISA) and immunochemiluminescent assays, respectively. HRT resulted in a significant decrease of Kupperman index, an increase of E2 and a decrease of FSH levels. These changes were associated with a significant decrease of NK cell cytotoxicity, IL-2 and IFN-gamma production. The levels of IL-4 production remained unchanged. Changes of NK cell cytotoxicity and cytokine release in individual patients did not correlate with changes of serum sex hormone levels. Nevertheless, the present results imply strongly that estrogen/progesterone HRT may affect cell-mediated immunity, thus being a potential factor influencing development and course of autoimmune disorders and neoplastic diseases.