During a 2-year prospective investigation of prolonged pregnancy in 727 women, 59 (8%) were delivered by cesarean section for fetal distress. This condition was diagnosed by means of electronic fetal heart rate monitoring in 47 of the women, and the patterns were unexpectedly characteristic of umbilical cord compression rather than uteroplacental insufficiency. Blinded sonar examinations were performed in 213 women, and the incidence of cesarean section for fetal distress as now described was significantly increased in those with oligohydramnios (two or fewer 1 cm pockets of amniotic fluid). We conclude that the pathophysiology of fetal distress in prolonged pregnancy is typically oligohydramnios that leads to compromised umbilical cord perfusion, rather than uteroplacental insufficiency.