The purpose of this study was to evaluate the effect of increasing birth weight on the success rates for a trial of labor in women with one previous cesarean delivery. To evaluate the effect of increasing birth weight for women who undergo a trial of labor, the medical records of women who had attempted a vaginal birth after cesarean delivery (VBAC) from 1995 through 1999 in 16 community and university hospitals were reviewed retrospectively by trained abstractors. Information was collected about demographics, medical history, obstetric history, neonatal birth weight, complications, treatment, and outcome of the index pregnancy. The analysis was limited to women with singleton gestations with a history of 1 previous cesarean delivery. Because women with previous vaginal deliveries have higher vaginal birth after cesarean delivery success rates, the women were divided into four risk groups on the basis of their birth history. Groups were defined as women with no previous vaginal deliveries (group 1), women with a history of a previous vaginal birth after cesarean delivery (group 2), women with a history of a vaginal delivery before their cesarean delivery (group 3), and a group of women with a vaginal delivery both before and after the previous cesarean delivery (group 4). There were 9960 women with a singleton gestation and a history of one previous cesarean delivery. The overall vaginal birth after cesarean delivery success rate for the cohort was 74%. The overall vaginal birth after cesarean delivery success rates for groups 1, 2, 3, and 4 were 65%, 94%, 83%, and 93%, respectively. An analysis of neonatal birth weights of <4000 g, 4000 to 4249 g, 4250 to 4500 g, and >4500 g in group 1 showed a reduction in vaginal birth after cesarean delivery success rates from 68%, 52%, 45%, and 38%, respectively. In the remaining groups, there was no success rate below 63% for any of the birth weight strata. For group 1, vaginal birth after cesarean delivery success rates were decreased when the indication for the previous cesarean delivery was cephalopelvic disproportion or failure to progress or when the treatment was either an induction or augmentation of labor. The uterine rupture rate was higher in women for group 1 with birth weights of > or =4000 g (relative risk, 2.3; P <.001). Women with macrosomic fetuses and a history of a previous vaginal delivery should be informed of the favorable vaginal birth after cesarean delivery success rates. Given the risks of vaginal birth after cesarean delivery, those women with no history of a vaginal delivery should be counseled that the success rates may be <50% when the neonatal birth weight exceeds 4000 g and that the success rates may be even lower if the indication for the previous cesarean delivery was cephalopelvic disproportion or failure to progress or if the treatment requires either induction or augmentation of labor. The uterine rupture rate was 3.6% in women for group 1 with a birth weight > or =4000 g.