Objective To investigate the role of nuclear factor-kappa B(NF-κB) in preterm birth with subclinical chorioamnionitis.Methods From October 2005 to October 2006,111 cases including 36 cases of preterm birth in labor,37 cases of full term gravida with spontaneous labor and 38 cases of full term gravida without threatened labor in the Hunan Province People's Hospital,third Xiangya Hospital of Central South University and Changsha Maternal and Child Care Service Center were enrolled in the study.After delivery,by pathology results of fetal membrane they were divided into two groups:subclinical chorioamnionitis group(subclinical infectious group ) and non-infectious group. Immunohistochemical staining and RT-PCR were used to observe the change of the p65 subunit of NF-κB family in maternal blood and fetal membrane in subclinical infectious group and non-infectious group.Results (1) The incidence of subclinical chorioamnionitis:there were 24 cases of subclinical chofioamnionitis in the 36 cases of preterm birth in labor(67% ),7 cases in the 37 cases of full term gravida with spontaneous labor group (19%)and 3 cases in the 38 cases of full term gravida without threatened labor group(8%).There was a significant difference among the three groups (P＜0.01). In the totally 111 cases,34 eases were classified as subclinical infectious group and 77 cases as non-infectious group.(2) In fetal membrane,the median of the average staining intensity of NF-κB p65 protein was higher in the subclinical chorioamnionitis group (8.0)than those in non-infectious group(4.0).Similarly,the average staining intensity of NF-κB p65 mRNA was higher in the subclinical infectious group (47.5±17.2) than those in non-infectious group(31.3±13.6).There was a significant difference between the two groups (P＜0.01).(3)In maternal blood,the expression of NF-KB p65 protein and mRNA was higher in subclinical chorioamnionitis group (8.0 and 42.6) than these in non-infectious group(4.0 and 23.6).There was a significant difference between the two groups (P＜0.01).(4) The concentration of NF-κB p65 protein in fetal membrane was positively correlated with that of maternal blood(r=0.581,P＜0.01 ) and the concentration of NF-KB p65 mRNA in fetal membrane was positively correlated with that of maternal blood (r=0.571,P＜0.01 ).Condusion The expression of NF-KB in maternal blood and fetal membrane in preterm birth with subclinical chorioamnionitis is higher and the two are correlated with each other.NF-KB p65 could be an accurate biochemical marker in predicting subclinical chorioamnionitis in preterm birth. NF-KB p65 plays an important role in the pathogenesis of subclinical chorioamnionitis in preterm birth.