Endometriosis requires estrogen for development and growth. Levels of estrogen receptor beta (ERβ) in endometriosis are reported to be 100 times higher than normal endometrial tissue and inhibition of ERβ activity by an ERβ selective antagonist suppresses endometriotic lesion growth in mice.1,2 Here, we assessed the role of ERβ in the development of the early endometriotic lesion by examining the effect of ERβ knock down on endometrial stromal cell (ESC) cell attachment and invasion. In vitro study. ERβ was knocked-down (ERβKD) with lipofectamine using shRNA in an immortalized human ESC line (tHESC). Western blot and densitometric analysis were used to assess protein expression. Established in vitro assays were used to assess WT and ERβKD tHESC attachment to peritoneal mesothelial cells (PMCs) and invasion through a monolayer of PMCs. Results were analyzed with unpaired t-tests. tHESCs with ERβKD demonstrated decreased protein expression by Western blot and densitometric analysis. ERβKD attachment to PMCs decreased in comparison to WT (42% vs 67%, respectively; p<0.001). ERβKD invasion through PMCs decreased in ERβKD compared to WT (264 ±16 vs 404 ± 22, respectively; p<0.005). ERβKD decreases tHESCs adhesion to and invasion through PMCs. ERβ may serve as a future therapeutic target to decrease early endometriotic lesion formation.