The clinical relevance of histologic evidence of acute ascending intrauterine infection has been called into question by descriptions of "silent" chorioamnionitis. The described frequencies of silent chorioamnionitis in normal and abnormal pregnancies vary widely because of differences in the definition of a normal pregnancy, methods of placental examination, and pathologic criteria. Therefore, we examined placentas from 161 uncomplicated gestations for the presence and severity of acute inflammation in the amnion, chorion-decidua, chorionic plate, and umbilical cord using strict gross and microscopic protocols. Indicators of amniotic fluid infection, specifically umbilical cord inflammation, amnionitis, and inflammation within the chorionic plate were present in 0, 1.2, and 4% of the cases, respectively. Silent chorioamnionitis was rare. There was a statistical association between the presence of acute inflammation and the occurrence of labor at term. Methods of tissue sampling that included a more extensive examination of the site of membrane rupture resulted in an increased frequency of diagnosis of acute inflammation at the site of rupture in vaginal deliveries at term.