A double-blind, placebo-controlled, prospective investigation was undertaken to determine whether the outpatient administration of prostaglandin E2 gel was helpful for ripening the cervix in postdate pregnancies. One hundred eighteen women with an uncomplicated pregnancy at or beyond 42 weeks' gestation with an unripe cervix (Bishop score less than or equal to 5) were randomly administered a single dose of gel containing either 2.5 mg prostaglandin E2 (n = 55) or a placebo (n = 63) before induction of labor with Pitocin. No side effects were detected in these healthy mothers and fetuses. A distinct change in Bishop score after 12 hours occurred more often in the prostaglandin E2 than in the placebo group (42% versus 6%, p less than 0.0001). Forty-four women (80%) who had received prostaglandin E2 were admitted in early labor; they required little or no oxytocin for augmentation. The duration of labor and maximum dose of oxytocin infused were significantly decreased in the prostaglandin E2 group, and forceps delivery or primary cesarean sections were performed less often when prostaglandin E2 was used (24% versus 44%, p less than 0.05). The outpatient administration of a single dose of prostaglandin E2 gel is safe in the uncomplicated postdate pregnancy and was found to significantly change the unripe cervix, enhance the onset of labor, minimize the need for oxytocin administration, and encourage a spontaneous vaginal delivery.