A prospective study was undertaken to evaluate the risk of uterine rupture or dehiscence after cesarean section. During the 10 years of the study, 24,644 patients were delivered of infants. Of these women, 2036 (8.3%) had previously undergone cesarean section. A trial of labor was allowed in 1008 of these patients, and 92.2% were delivered vaginally. The incidence of uterine rupture in this trial of labor group was 0.6%, compared with 0.4% in the total group. Cesarean section scar rupture caused no serious complications in either the mothers or the offspring in the trial of labor group. Uterine rupture in this group was not associated with use of oxytocin or epidural analgesia. Patients with lower-segment scar rupture had no history of puerperal pyrexia. The incidence of uterine dehiscence was 4%. In summary, the risk of uterine rupture in patients who have previously undergone cesarean section but are allowed a trial of labor is low and not associated with serious complications. Vaginal delivery is therefore considered the safest route of delivery in these patients.