Context:The human endometrium is a complex multicellular tissue subject to cyclical fluctuations in ovarian-derived steroid hormones. Fertile cycles are characterised by differentiation (decidualization) of endometrial stromal cells.Objective:To determine the impact of human stromal cell decidualization on biosynthesis and secretion of estrogens.Design:Primary cell culture, cells were decidualized in vitro. Some were treated with an aromatase inhibitor.Setting:A University Research Institute.Patients:Primary endometrial stromal cells (ESC) were derived from women with normal menstrual cycles (n=12). None of the women were receiving hormonal therapy or suffering from endometriosis.Main outcome measurements:Expression of mRNA and protein encoded by the aromatase (CYP19A1) and 3βHSD (HSD3B1) genes was assessed. Aromatase activity was measured using a tritiated water assay; steroid metabolism was determined using thin layer chromatography (TLC). Estrone (E1) and estradiol (E2) in cell culture media were measured by ELISA.Results:Decidualization induced a 2-fold increase in aromatase mRNA. Aromatase protein was only detected in decidualized ESC. 3βHSD protein was present in ESC both before and after decidualization; concentrations appeared unchanged. The existence of functional aromatase in decidualized ESC was confirmed; E1 and E2 were secreted into culture media in decidualized ESC; concentrations were reduced when cells were incubated with an aromatase inhibitor. Decidualization resulted in reduced metabolism of E2 and an increase in the ratio of E2:E1.Conclusion:Decidualization is characterised by an increase in aromatase expression/activity favouring the generation of an E2-dominated estrogen microenvironment within the endometrial stroma.