We evaluated Inter-alpha inhibitor proteins (IaIp) as a diagnostic marker in neonatal sepsis. Samples were collected from 573 neonates who were examined for suspected sepsis. IaIp level was significantly lower in the septic group (121+/-71 mg/L) than in the non-septic group (322+/-91 mg/L). The optimal cutoff value with the receiver operating characteristic curve was <or= 177 mg/L (sensitivity, 89.5%; specificity, 99%; positive predictive value, 95%; negative predictive value, 98%) with area under the curve of 0.94. IaIp is a more reliable diagnostic marker for neonatal sepsis than other available tests.