The vaginal microflora of 49 women in idiopathic preterm labor was compared with that of 38 term controls to determine whether the presence of bacterial vaginosis (BV) and/or specific microorganisms would influence the rate of preterm delivery. Demographic factors, pregnancy outcome, and reproductive history were also studied. BV, as defined by the presence of clue cells in a vaginal wet mount and characteristic microbial findings in a stained vaginal smear and vaginal culture, was more common in women with preterm labor and delivery than in controls (P < 0.01). The condition, diagnosed in 41% of women who had both preterm labor and delivery (n = 22) and in 11% each of women who had preterm labor but term delivery (n = 27) and controls, was associated with a 2.1-fold risk (95% confidence intervals, 1.2 to 3.7) for preterm birth prior to 37 weeks of gestation. BV was associated with low birth weight. Of 49 women with preterm labor, 67% (8 of 12) of women with BV were delivered of low-birth-weight neonates (< 2,500 g) compared with 22% (8 of 37) of women without the condition (P < 0.0005). The presence of hydrogen peroxide-producing facultative Lactobacillus spp. was strongly negatively associated with both preterm delivery and BV. BV-associated microorganisms, i.e., Mobiluncus, Prevotella, and Peptostreptococcus species, Porphyromonas asaccharolytica, Fusobacterium nucleatum, Mycoplasma hominis, and high numbers of Gardnerella vaginalis were significantly associated with preterm delivery; all species also strongly associated with BV (P = 0.0001 for each comparison). Mobiluncus curtisii and Fusobacterium nucleatum were recovered exclusively from women with preterm delivery. Our study clearly indicates that BV and its associated organisms are correlated with idiopathic premature delivery.