To identify factors associated with a vaginal second delivery in women who had one previous cesarean section. A nested case-control study was carried out as a secondary data analysis of an original retrospective, population-based cohort study of women who delivered their first child during 1985 in the city of Campinas, São Paulo, Brazil, and who were interviewed 10 years later in 1995. The study population consisted of 1352 women who had their first delivery by cesarean section and who had also had at least one subsequent delivery. The group of cases (150 women, around 11% of the sample) consisted of women who had a vaginal second delivery, and the control group was made up of 1202 women who had a cesarean section at second delivery. For each possible associated factor we calculated the odds ratio and 95% confidence interval. For ordered categorical variables the c2 test for trend was used. Unconditional multivariate regression analysis was used to estimate the adjusted odds ratio for each associated factor. The factors significantly associated with vaginal delivery were monthly family income below 5-fold the Brazilian minimum monthly wage, reliance on the Brazilian national health system for healthcare, low maternal age, and first cesarean section indicated because of fetal breech or transverse presentation, or twin pregnancy. Among those women who also had a cesarean section at their second delivery, only 11% had undergone a trial of labor. The main determining factors for a vaginal second delivery in women with a previous cesarean section were unfavorable social and economic factors.18107-13