Increased concentrations of activated pelvic macrophages and lymphocytes and the elevated levels of specific cytokines and growth factors reviewed earlier support the hypothesis that the immune response is associated intricately with endometriosis. Whether components of the innate and adaptive immune systems have a primary, causative role or merely reflect a passive reaction to the presence of ectopic implants is unknown. The precise roles of the soluble peritoneal factors also are unknown. The authors propose that a complex network of locally produced cytokines modulate the growth and inflammatory behavior of endometriosis (see Fig. 1), including implant proliferation and invasion, recruitment of capillaries to the growing lesions, and further chemoattraction of leukocytes to foci of peritoneal inflammation. Clinicians who seek new treatments for endometriosis should consider inflammatory mediators as potential targets.