The use of combined hormone replacement therapy (HAT) with oestrogens and progestins in postmenopausal women has been associated with an increased risk for developing breast cancer. The reasons are not fully understood, but influence of HRT on endogenous conversion of female sex hormones may be involved. The expression of 17 beta hydroxysteroid dehydrogenases (17 beta HSD), which are enzymes catalysing the conversion between more or less potent oestrogens, may partly be regulated by progestins. The breast cancer cell lines T47D, MCF7 and ZR75-1 were treated with progesterone, medroxyprogesterone acetate (MPA) or levonorgestrel for 48 and 72 h at 10(-7) and 10(-9) M to investigate influence on 17 beta HSD1, 17 beta HSD2 and 17 beta HSD5 mRNA expression measured by real time PCR. The expression of 17 beta HSD1 increased in progesterone and levonorgestrel treated T47D cells (48 h 10(-7) M P = 0.002; P < 0.001) and 17 beta HSD5 increased after progesterone treatment (48 h 10(-7) M P = 0.003), whereas the expression of 17 beta HSD2 decreased after the (48 h 10(-7) M P = 0.003; P < 0.001). Similar, but less prominent effects were seen in MCF7 and ZR75-1. The progestin effects on 17 beta HSD-expression were lost when T47D cells were co-treated with progestins and the progesterone receptor (PgR) inhibitor mifprestone. We show that both reductive (17 beta HSD1 and 17 beta HSD5) and oxidative (17 beta HSD2) members of the 17 beta HSD-family are under control of progesterone and progestins in breast cancer cell lines. This is most clear in T47D cells which have high PgR expression. 17 beta HSD-enzymes are important players in the regulation of sex steroids locally in breast tumours and tumoural expression of various 17 beta HSD-enzymes have prognostic and treatment predictive relevance. We propose a mechanism for increased breast cancer risk after HRT in which hormone replacement affects the expression of 17 beta HSD-enzymes, favouring the expression of reductive enzymes, which in turn could increase levels of bioactive and mitogenic estrogens in local tissue, e.g. breast tissue. (C) 2012 Elsevier Inc. All rights reserved.