We prospectively studied the relationship of pregnancy outcome to bacterial vaginosis, an anaerobic vaginal condition, and to other selected genital pathogens among 534 gravid women. Bacterial vaginosis was presumptively diagnosed by gas-liquid chromatographic identification of microbial organic acid metabolites in 102 women (19%), and cervical infection with Chlamydia trachomatis was found in 47 (9%) of the women. Although women with and without bacterial vaginosis had similar demographic and obstetric factors, neonates born to women with bacterial vaginosis had lower mean birth weight than did neonates born to women without bacterial vaginosis (2960±847 g vs 3184 ±758 g). Bacterial vaginosis was significantly associated with preterm premature rupture of the membranes (odds ratio [OR], 2.0; 95% confidence interval [CI], 1.1 to 3.7), preterm labor (OR, 2.0; CI, 1.1 to 3.5), and amniotic fluid infection (OR, 2.7; CI, 1.1 to 6.1), but not with birth weight below 2500 g (OR, 1.5; CI, 0.8 to 2.0). Cervical infection with C trachomatis was independently associated with preterm premature rupture of the membranes, preterm labor, and low birth weight (OR, 1.5; CI, 0.8 to 2.0).