The obstetric management of 340 pregnancies delivered 2 or more weeks postterm and 1408 pregnancies delivered at 41 weeks is compared with that of 5915 pregnancies delivered at 39 to 40 weeks. In all patients menstrual dating was confirmed by early ultrasonographic examination. Postterm labor in primiparas resulted in a high cesarean section rate because of failure to progress; this increased rate was observed even after controlling for induction of labor and the size of the infant. We suggest that primiparas who go postterm often have increased uterine dysfunction. Uterine dysfunction accounts for the increase in the cesarean section rate and is a partial explanation for "failed" inductions.