Endometriosis is a disorder affecting 5-10% of women in which endometrial fragments implant at ectopic sites, causing pain and infertility. Mouse models and human studies demonstrate that inflammation is pivotal to the pathogenesis of endometriosis1. Seminal plasma contains an abundance of signalling molecules that can activate immunomodulatory pathways in reproductive tract epithelial cells post-coitus2. We hypothesise that exposure to seminal plasma promotes the production of pro-inflammatory cytokines in endometrial tissue, and that this facilitates adhesion and growth of ectopic endometriotic lesions. To test this, we determined levels of interleukin (IL)-6, IL-8 and GM-CSF in conditioned media of immortalised human endometrial stromal cells (iHESC) after exposure to 0.08–10% human seminal plasma for 2–24 hours by ELISA. Using a xenograft model, in which human endometrial tissue fragments are implanted subcutaneously in SCID mice, we then examined the effect of seminal plasma on lesion development in vivo under two paradigms: (i) direct exposure of the tissue to seminal plasma prior to implantation, and (ii) i.p injection of seminal plasma (to mimic the systemic post-coital inflammatory response). The results demonstrated that incubation of iHESC with seminal plasma for 6 hours induces significant (P<0.005 by 1-way ANOVA) increases in GM-CSF, IL-6 & IL-8 concentrations in the media in dose-dependent fashion. In vivo, direct exposure of endometrial tissue, but not i.p injection of seminal plasma, results in larger endometriotic-like lesions (n≥3; P<0.05 by independent samples median test) in mice. Overall, the data support the hypothesis that exposure to seminal plasma favours the development of endometriotic lesions. The mechanism may involve enhanced immune cell recruitment to the implantation site, secondary to elevated cytokine levels in the endometrial tissue. This finding has clinical significance insofar as simple strategies such as perimenstrual abstinence and barrier contraception may prevent endometriotic lesion formation or development in some women.