The impact of chorioamnionitis on the course of labor is controversial. Some clinicians believe the infection has stimulatory effects, whereas others suspect inhibitory influences. Two hundred sixty-six pregnancies with chorioamnionitis requiring labor stimulation with oxytocin were matched to uninfected women for maternal age, race, parity, gestational age, oxytocin dosage regimen, indication for labor stimulation, type of labor stimulation, cervical dilatation at initiation of oxytocin, and time for rupture of membranes to initiation of labor stimulation. Chorioamnionitis diagnosed before oxytocin infusion was associated with shorter oxytocin initiation-to-delivery intervals (4.3 versus 5.6 hours; P = .04) and had no significant impact on the cesarean rate compared with matched controls. In contrast, pregnancies complicated by chorioamnionitis detected late in labor were associated with markedly longer oxytocin initiation-to-delivery intervals (12.6 versus 7.9 hours; P less than .0001) and a fourfold increase in cesarean for dystocia compared with matched controls (40 versus 10%; P less than .0001). Thus, the impact of chorioamnionitis on the course of labor can be divided into two clinical presentations. That diagnosed before labor stimulation does not increase the use of cesarean, whereas that diagnosed after oxytocin stimulation may be a sign of abnormal labor, as it was associated with a marked increase in abdominal delivery for dystocia.