Bacterial vaginosis has been implicated as a cause of acute chorioamnionitis and preterm delivery. This study was designed to determine any association between the detection of bacterial vaginosis on a prenatal Papanicolaou (Pap) smear, defined as a shift in vaginal flora, and the subsequent occurrence of acute chorioamnionitis or preterm labor. A 47-mo retrospective case-control analysis comparing 186 patients with histologically-proven acute chorioamnionitis (cases) and 186 controls was performed. Initial prenatal Pap smears were evaluated for the presence of altered vaginal flora. Pap smears from women with acute chorioamnionitis were more likely to have altered vaginal flora than those without chorioamnionitis (P < 0.01). Preterm delivery was also more common among women with a shift in vaginal flora (P < 0.01). In conclusion, women at risk for chorioamnionitis and/or preterm delivery may be able to be identified by the results of their prenatal Pap smear.