Bacterial species associated with bacterial vaginosis have been isolated more frequently from endometrial cultures of patients with postpartum endometritis than expected from the prevalence of bacterial vaginosis among pregnant women. To further assess the association between bacterial vaginosis and postpartum endometritis, vaginal Gram smears were obtained from women admitted for delivery. Vaginal smears of women delivered by cesarean were scored as normal or as indicating bacterial vaginosis. Factors related independently to postpartum endometritis by multiple logistic regression analysis included maternal age less than 25 years, any duration of membrane rupture, and bacterial vaginosis. The unadjusted odds ratio for the development of postpartum endometritis associated with bacterial vaginosis (odds ratio = 6.1, 95% confidence interval 3.3-15.9) was not appreciably changed in the multivariable analysis (odds ratio = 5.8, 95% confidence interval 3.0-10.9) after adjusting for maternal age, duration of labor, and duration of membrane rupture. At the time of endometritis, Bacteroides sp, Peptostreptococcus sp, and Gardnerella vaginalis were isolated more frequently from the endometrium using a triple lumen endometrial sampling method among patients with bacterial vaginosis than among those with a normal Gram stain. Bacterial vaginosis appears to be an important risk factor for postpartum endometritis after cesarean delivery.