Objective To explore the correlation between neutrophil-to-lymphocyte ratio(NLR)and Behcet's disease(BD)activity.Methods A total of 103 BD patients were divided into the low activity group(0-4,61 cases)and the high activity group(5-11,42 cases)according to electronic medical record-based disease activity index(EMRAI)score.The white blood cell(WBC),neutrophil(NEU),lymphocyte(LY),platelet(PLT),red blood cell(RBC),hemoglobin(Hb),erythrocyte sedimentation rate(ESR),C-reactive protein(CRP),IgG,IgA,IgM,complement C3 and C4 were detected.NLR and platelet-to-lymphocyte ratio(PLR)were calculated.The correlation between NLR,PLR and ESR,CRP,EMRAI were analyzed.Logistic regression was used to analyze the influencing factors of BD disease activity.Receiver operating characteristic(ROC)curve was drawn to evaluate the effectiveness of NLR in judging BD disease activity.Results WBC,NEU,PLT,ESR,CRP,NLR,PLR,complement C3 and C4 in patients were higher in the high activity group than those in the low activity group(P＜0.05),and there were no significant differences in other indexes(P＞0.05).NLR was positively correlated with ESR,CRP and EMRAI in the whole group,while PLR was positively correlated with ESR,CRP and EMRAI in the whole group(P＜0.05).Logistic regression analysis showed that high NLR was a risk factor for BD disease activity(OR=1.511,95%CI:1.080-2.113,P＜0.05).ROC curve analysis showed that the area under the curve(AUC)of NLR in evaluating BD disease activity was 0.706(95%CI:0.603-0.809).Conclusion NLR is effective in judging the disease activity of BD patients,and can be used as a biological index to evaluate the disease activity of BD.