Objective : The effects of plasma and peritoneal fluid from endometriosis patients on apoptosis of neutrophils were investigated. Methods : Neutrophils and plasma were obtained from women with endometriosis (n=20) and healthy control donors (n=20). Peritoneal fluid from endometriosis patients (n=10) and the controls (n=5) were collected prior to laparoscopic operation. Apoptotic changes of the cells were evaluated by morphological changes using Giemsa staining. DNA electrophoretic analysis were used to confirm apoptosis. Results : Compared to the plasma of healthy controls, the addition of 10% plasma from patients with endometriosis to culture medium reduced the percentage of apoptotic cells from 65.3±6.6 % to 27.2±4.6% in cultures of neutrophils maintained in vitro at 24 h. The addition of 10% peritoneal fluid obtained from patients with endometriosis to culture medium also inhibited the apoptosis of neutrophils compared with control group (10.5±4.3% vs. 45.3±4.8%). 10 ng of interleukin-8 (IL-8) in the presence of plasma from controls and endometriosis patients further delayed the spontaneous apoptosis of neutrophils. Neutralizing IL-8 antibody abrogated the delay of apoptosis of neutrophils induced by peritoneal fluid but not in the plasma of endometriosis patients. Neutrophils obtained from patients with endometriosis showed delayed apoptosis in the absence and in the presence of plasma or peritoneal fluid from either source and this delay was not abrogated by neutralizing IL-8 antibody. Conclusion : These findings show that IL-8 is one of the neutrophil survival factors in the peritoneal fluid of endometriosis patients, which contains both pro-apoptotic and anti-apoptotic cytokines. However, our results indicate that an unidentified survival factor is present in the plasma of patients with endometriosis, and also suggest that the impairment of innate immunity, such as the delayed apoptosis of neutrophils may be related with pathogenesis of endometriosis.