During the establishment of pregnancy, the endometrium undergoes dynamic remodelling in order to establish a ‘receptive’ microenvironment. Decidualisation, a key part of this process, is characterised by differentiation of endometrial stromal fibroblasts which secrete growth factors and cytokines that regulate vascular remodelling and immune cell influx, processes that are essential for implantation and placental development. Recent studies in our laboratory have revealed that decidualisation results in altered expression of enzymes that regulate biosynthesis and metabolism of estrogens. We believe that intra-tissue steroid production may play an important regulatory role in the endometrium during the establishment of pregnancy. In the present study, we tested the hypothesis that changes in the availability of bioactive androgens impact on uterine function during decidualisation.Primary human endometrial stromal cells were isolated from endometrial biopsies collected from women during the proliferative phase of the cycle (n=20). In vitro decidualisation was induced by treatment with progesterone and cAMP. The expression of androgen biosynthetic enzymes was assessed by qPCR, Western Blot and immunocytochemistry. Concentrations of the decidualisation marker IGFBP-1 and testosterone (T) and dihydrotestosterone (DHT) were determined by ELISA. We found that decidualisation was associated with biosynthesis of androgens. Time-dependent changes in the expression of androgen biosynthetic enzymes AKR1C3 and SRD5A1 were detected by qPCR (n=8 patients, p<0.01) and Western blot. Decidualisation was associated with secretion of the androgen receptor agonists T and DHT (n=8, p<0.001). Androgen action was inhibited by co-treatment with the antiandrogen flutamide which significantly reduced secretion of the decidualisation marker IGFBP-1 (n=8, p<0.01) and altered the expression of receptivity genes such as SPP1 (n=8, p<0.001). These data suggest intra-uterine androgens may be critical for decidualisation and endometrial receptivity. We speculate that decidualisation is associated with a unique steroid microenvironment that may play a critical role by ‘fine tuning’ the endometrium in preparation for pregnancy.