Background Currently,all the diagnostic indicators for endometriosis lack perfect sensitivity and specificity.According to the characteristic of endometriosis,we analyzed the new biomarker neutrophil-to-lymphocyte ratio (NLR) and the combination of NLR and serum CA-125 to investigate their diagnostic value for identifying stages Ⅲ and Ⅳ endometriosis.Methods The values of serum CA-125 and routine blood tests were collected from 197 patients with endometriosis,102 with benign tumors and 112 healthy individuals.We investigated the sensitivity and specificity of NLR and its combination with serum-CA-125 for diagnosing stages Ⅲ and Ⅳ endometriosis by using receiver operating characteristic (ROC).Results The mean values of NLR,the combination of serum CA-125 and NLR (combination) of the groups with stages Ⅲ and Ⅳ endometriosis were significantly higher than the other two groups.Serum CA-125,NLR,and the combined biomarkers could significantly discriminate the stages Ⅲ and Ⅳ endometriosis group from the other two groups (P＜0.05).NLR shows a lower sensitivity of 57.9％ and specificity of 65.2％ with a cutoff value at 1.82.And the combination of biomarkers has the highest AUC of 0.949 with a sensitivity of 86.8％ and specificity of 92.0％ at the cutoff value of 44.40.In addition,for patients with negative CA-125,55.36％ and 53.57％ of the patients were able to be diagnosed with endometriosis by using NLR alone and the combination of biomarkers.Conclusion For diagnosing stages Ⅲ and Ⅳ endometriosis,the neutrophil-to-lymphocyte ratio is a better adjuvant to serum CA-125,and the neutrophil-to-lymphocyte ratio is valuable in diagnosing stages Ⅲ and Ⅳ endometriosis for patients with negative serum CA-125.