The purpose of this study was to determine the importance of varying histologic stages of inflammation in the placental membranes and cord. Acute inflammation was histologically staged in fetal membranes and umbilical cord sections from 2899 placentas received from consecutive singleton deliveries. Then clinical data were collected for a subset of randomly selected placentas with stage 1 through stage 4 membrane inflammation (n = 212) and without significant inflammation (stage 0, n = 216). Statistical analyses revealed that increasing stage of membrane inflammation was associated with an increasing rate of funisitis, perinatal death, and preterm birth (P < .05). Inflammation permeating the entire trophoblastic layer of the chorion (stage 1) was associated with clinical symptoms of intrauterine infection and thus was an important pathologic finding. Acute necrotizing chorioamnionitis was very strongly associated with perinatal death and preterm birth. Acute funisitis was a more specific but less sensitive marker for perinatal complications than inflammation in the membranes. With increasing stage of funisitis, there was an increased incidence of clinical symptoms of intrauterine infection, preterm birth, and perinatal death. Almost three-fourths of the cases with histologic evidence of membrane inflammation were clinically silent. In conclusion, increasing histologic stages of inflammation of the membranes and cord are associated with an increased rate of perinatal morbidity and mortality. Stage I membrane inflammation provides a clinically acceptable minimum threshold for the reporting of pathologic changes.