Peritoneal fluid (PF) was studied for the presence of endometrial tissue in a consecutive series of 67 women (with documented tubal patency) undergoing diagnostic laparoscopy, tubal lavage, and hysteroscopy. PF was completely aspirated from the cul-de-sac both before and after uterine irrigation. The PF was then analyzed for the presence of endometrial tissue. In native PF no significant difference in the incidence of endometrial tissue between patients with (19%) and without (11%) endometriosis (P = 0.6) was observed. Refluxed PF, obtained after uterine irrigation, showed a significantly higher incidence of endometrial tissue in women with endometriosis (76%) as compared to controls (42%) (P = 0.03). We propose two models to explain the development of endometriosis. These are not mutually exclusive, may be independent of each other, and may represent two distinct pathophysiologic disease processes.