Objective : Development of endometriosis is associated with endometrial stromal cell proliferation. The effects of nitric oxide (NO) on the proliferation of endometrial cells and the effects of peritoneal fluid from women with and without endometriosis on the production of NO and NO-induced changes were investigated. Methods : Endometrial stromal cells were prepared from women with endometriosis (n=10) and healthy control (n=10). Peritoneal fluid from 10 patients were collected at the beginning of laparoscopic procedures. The proliferation of cells was assessed by [3H]thymidine. Nitrite, a stable NO product, was measured with Griess reagents. Expression level of endothelial nitric oxide synthase (eNOS) was analyzed by quantitative RT-PCR. Results : Sodium nitroprusside, an NO donor, reduced proliferation of the endometrial and endometriosis cells from primary culture in a dose-dependent manner. The increased production of NO and inhibitory effect of NO on the endometrial stromal cell proliferation were reduced by peritoneal fluid from patients with endometriosis but not by interleukin-8, 17-β estradiol or TGF-β1. Transcripts of the endothelial NOS (eNOS) gene were detected in unstimulated endometrial cells and the expression patterns were not changed by exposure of cells to SNP or peritoneal fluid for 24 hours. Conclusion : It is postulated that these findings show NO inhibits proliferation of endometrial cells and the inhibitory effect of NO on endometrial cells was abrogated by certain factor (s) of peritoneal fluid from patients with endometriosis. In conclusion, NO appears to be involved in the pathogenesis of endometriosis.