During the past decade, macrophage-derived substances such as prostanoids, cytokines, growth factors and angiogenic factors have been detected in the peritoneal fluid of women with endometriosis. In particular, growth-promoting and angiogenic factors are considered to be substantially involved in the pathogenesis of endometriosis. In this study, vascular endothelial growth factor (VEGF), transforming growth factor β (TGF-β) and intercellular adhesion molecule 1 (ICAM-1), substances recently detected in the peritoneal fluid of women with endometriosis, were assessed with regard to their concentrations in different stages of endometriosis and changes of the peritoneal paracrine activity after medical treatment with a gonadotrophin releasing hormone agonist (GnRHa). Peritoneal fluid was obtained from patients with endometriosis during laparoscopy before and after a 4-month treatment with a GnRHa. VEGF, TGF-β and ICAM-1 could be detected in all women presenting with various stages of active endometriosis. After GnRHa therapy, all patients showed significant decreases in mean concentrations of VEGF (194 ± 77 pg/ml), TGF-β (902 ± 273 pg/ml) and ICAM-1 (157 ± 52 ng/ml). Patients with stage III and IV endometriosis (according to the rAFS score) had much higher concentrations of VEGF and TGF-β before treatment compared with those patients with mild endometriosis (rAFS stages I and II). The most striking decrease in concentration was for TGF-β, from 902 pg/ml before to 273 pg/ml after therapy. These results indicate an important role for paracrine activity in the establishment and maintenance of endometriosis. Indeed, treatment with a GnRHa may reduce paracrine activity in the peritoneal cavity via hypo-oestrogenism and provide proof of successful therapy.