We performed a two-part study to determine relationships of bacterial vaginosis and intraamniotic infection. In the first part of the study, we determined whether bacterial vaginosis organisms (Gardnerella vaginalis, Mycoplasma hominis, and anaerobes) were associated with each other in the amniotic fluid of 408 cases of intraamniotic infection. In the second part, we determined the association of bacterial vaginosis itself with intraamniotic infection in 125 cases at high risk for intraamniotic infection. Strong associations were observed among the bacterial vaginosis organisms in amniotic fluid (p less than 0.01 to p less than 0.001). Women with bacterial vaginosis were more likely to develop intraamniotic infection than those without bacterial vaginosis (69% vs. 46%, p = 0.03). Women with bacterial vaginosis were more likely to have G. vaginalis and M. hominis in the amniotic fluid (p less than 0.01 and 0.04, respectively). These observations implicate bacterial vaginosis as a cause of intraamniotic infection.