Objective: To test for differences in the amount and activity pf peritoneal macrophages present in the peritoneal fluid of women with, and without endometriosis using prostaglandin release by macrophages in culture as a marker. Patients: Women of reproductive age undergoing laparoscopy for infertility or chronic pelvic pain with postoperative diagnosis of endometriosis and women undergoing laparoscopy for sterilization. Methods: Peritoneal fluid was aspirated during laparoscopy, volume was recorded, macrophages were isolated via a Ficoll Paque gradient and kept in primary culture. PGE(2) and PGF(2 alpha) release of the cells were measured before and after stimulation with zymosan. Results: Women with endometriosis had significantly more peritoneal macrophages than controls. Peritoneal macrophages of women with endometriosis released significantly more PGE, than those of the control group: 8.4 +/- 2.0 versus 1.4 +/- 0.4 ng/ml/10(6) cells (mean +/- SEM, p = 0.0005) and PGF(2 alpha): 10 +/- 4.3 (endometriosis) versus 1.8 +/- 0.4 (control) ng/ml/10(6) cells (mean +/- SEM, p = 0.045). Conclusion: There is a significant increase in the amount of prostaglandins released by peritoneal macrophages from women with endometriosis. These prostaglandins might alter uterine and tubal contractility, thereby affecting fertility.