Objective To evaluate the relationship between levels of soluble Fas(sFas)and soluble Fas ligand(sFasL)in serum and peritoneal fluid of endometriosis-associated infertility. Methods The soluble Fas ligand and soluble Fas levels in serum and peritoneal fluid of 20 infertile patients with endometriosis were assessed with enzyme-linked immunosorbent assay, and were compared with 14 infertile patients due to chronic pelvic infectious disease and 16 fertile controls. Results The sFasL levels were significantly higher in infertile patients with endometriosis(175.09 ± 80.55 pg/mL in serum and 284.50 ± 152.38 pg/mL in peritoneal fluid)than those of infertile controls (88.47 ± 43.55 pg/mL in serum and 17.30 ± 9.62 pg/mL in peritoneal fluid)and fertile controls(16.13 ± 11.75 pg/mL in serum and 8.84 ± 2.31 pg/mL in peritoneal fluid). In contrast, as for the sFas levels, infertile patients with endometriosis(828.60 ± 429.65 pg/mL in serum and 349.61 ± 288.89 pg/mL in peritoneal fluid)did not show any significant difference compared with those in infertile patients resulting from pelvic infectious disease(868.75 ± 570.48 pg/mL in serum and 181.76 ± 157.78 pg/mL in peritoneal fluid)and fertile control(822.26 ± 129.12 pg/mL in serum and 318.42 ± 145.16 pg/mL in peritoneal fluid). Conclusions Based upon these results, high level of sFasL in serum and peritoneal fluid and thus apoptosis mediated by it may be implicated in the mechanism involved in endometriosis-related infertility.