Context –Endometrial differentiation is required for embryo receptivity, placental angiogenesis and a successful pregnancy outcome. It also is critical to prevent the development of endometrial hyperplasia and cancer. Clinical conditions associated with inflammation and reduced fecundity (eg, endometriosis, recurrent pregnancy loss) manifest impaired decidualization. Objective – To understand decidualization defects in endometriosis, our laboratory has identified several differentially expressed endometrial genes whose functions appear to be critical for endometrial preparation for embryonic implantation. One of these encodes the gap junction subunit protein, connexin (Cx)43. Methods – Primary human endometrial stromal cells (ESC) were cultured from proliferative phase biopsies. Cx43 mRNA and protein and other decidual biomarkers were quantified by qRT-PCR and Western blots. ESC morphology and mesenchymal-epithelial transition (MET) indicators also were assessed. Participant(s) – Seven healthy, parous volunteers of reproductive age with normal menstrual cycles. Intervention(s) – ESC were exposed to 10 nM estradiol, 100 nM progesterone and 0.5 mM cyclic AMP for up to 14 days, followed by hormone withdrawal for 14 days, mimicking a biphasic ovulatory cycle. IL-1β was used to recapitulate effects of macrophage-derived cytokines. Main Outcome Measure(s) – Reversible ESC differentiation was documented following hormone treatment and withdrawal. Result(s) – Classical changes in cell shape and markers of MET were induced during hormonal decidualization, as was upregulation of Cx43. By contrast, IL-1β was found to dramatically inhibit Cx43 expression and other decidual biomarkers. Conclusions – Cx43-containing gap junctions regulate stromal decidualization, whereas the proinflammatory cytokine IL-1β disrupts these channels. The findings may have therapeutic implications for improving embryonic receptivity in women with inflammatory gynecological disorders.