Interleukin-8 (IL-8) exerts chemotactic activity on neutrophils at inflammatory sites to eliminate invading bacteria. To investigate whether the preterm fetus with chorioamnionitis produces IL-8, the titers of IL-8 were examined in sera of babies with (n = 38) and without chorioamnionitis (n = 34) using an EIA kit specific for IL-8. The infected babies had a significantly higher IL-8 titer than those not infected at 22-36 gestational weeks. The IL-8 titer was increased even in the mild histologic stage of chorioamnionitis and became much higher in the more severe stage. The IL-8 elevation, however, was remarkably suppressed by infusion of a steroid into the mother to promote fetal lung maturation. This retrospective study demonstrated that titration of IL-8 in cord serum is a more useful marker for the early detection of chorioamnionitis, because of its higher sensitivity and specificity, than conventional markers such as C-reactive protein.