Cesarean delivery has become the most frequently performed major operation in the United States. Widespread use of vaginal birth after previous cesarean delivery could potentially eliminate up to one-third of cesareans. However, many physicians have been reluctant to adopt this policy without large studies conclusively demonstrating its safety. This study evaluated the maternal and perinatal outcomes of over 5000 cases of labor after previous cesarean delivery. This multicenter study began in 1984 and initially included nine California hospitals. During the first 2 years, there were 1776 trials of labor resulting in 1314 vaginal births. In January 1986 two additional hospitals joined the collaborative project. Over the next 3 years, there were 3957 trials of labor resulting in 2977 vaginal births at the 11 participating hospitals. During the entire study period, 5733 patients opted for a trial of labor and 4291 (75%) delivered vaginally. There were no maternal deaths in the trial-of-labor group, and perinatal mortality was not significantly different from that of the general obstetric population. These results support the findings of numerous smaller studies that have concluded that the policy of routine repeat cesarean delivery should be abandoned.