Active endometriosis is characterized by hypervascularization both within and surrounding the implant; therefore the presence of angiogenic factors in the peritoneal environment would be of great importance. Vascular endothelial growth factor (VEGF) is a potent angiogenic factor involved in both physiological and pathological angiogenesis. We sought to determine if VEGF was present in the peritoneal fluid of women with and without endometriosis, and to establish if differences exist between these groups. VEGF was present in all patients sampled. The fluid from patients with endometriosis contained significantly greater amounts of VEGF than controls. Cyclic variations in VEGF concentration were seen in fluid from patients with endometriosis, the VEGF concentration in proliferative phase being significantly higher than in the secretory phase. The concentration of VEGF in this fluid was also significantly higher than that found in the proliferative and secretory phases of women without endometriosis. No cyclic variations in VEGF were seen in the control group. We suggest that elevated levels of VEGF in the peritoneal fluid of patients with endometriosis may be critical in the pathogenesis of endometriosis.