OBJECTIVE: To review and evaluate published studies that have examined the role of peritoneal fluid (PF) in the development of endometriosis. DESIGN: Important studies related to this topic have been identified through a computerized bibliographical search (MEDLINE), as well as by manually scanning the published literature of major reproductive journals over the last several years. MAIN OUTCOME MEASURES: Studies that examined the effect of PF or its components on the pathophysiology of endometriosis are discussed. These include reported effects on the histogenesis, maintenance and proliferation of endometriosis, as well as related actions on infertility. RESULTS: The majority of investigations into the role PF plays in the pathogenesis of endometriosis have evaluated how it may adversely affect fertility. Suggestive but inconclusive data in the literature indicate that degenerating endometrial tissue may release a biochemical factor(s) into the peritoneal environment that is capable of inducing ectopic endometrium formation. Peritoneal fluid itself contains growth factor that may play a role in the implantation and maintenance of the ectopic endometrium. CONCLUSIONS: The PF of women with endometriosis has been shown to contain angiogenic as well as other growth factors. These compounds may contribute to the proliferation of the ectopic endometrium.