Previous studies have indicated a higher prevalence of allergic disease among women with endometriosis. It is already well established that type 1 allergies develop in a Th2 cytokine environment. Recent studies have shown, however, that IL-25 induces a Th2 development of naive T lymphocytes and is central in the Th2 response. The aim of this case-control study was to investigate the presence of IL-25 in the peritoneal fluid of women suffering from endometriosis. PF was obtained both from women undergoing laparoscopic surgery due to endometriosis (25 cases) and from women wanting sterilisation (19 controls). IL-25 levels were then investigated by ELISA. Women with endometriosis showed significantly higher levels of IL-25 in their PF (p=0.019) compared to controls. IL-25 levels did not correlate with the stage of endometriosis. Both Th2-cells and mast cells express IL-25, which could favor the development of allergies by perpetuating a hypersensitivity reaction. Further, IL-25 may also hold a role as a diagnostic tool.