Objective: The purpose of this study was to determine the effectiveness of treatment over time for bacterial vaginosis in pregnancy and the probability of spontaneous resolution with placebo. Study design: Women with asymptomatic bacterial vaginosis on Gram stain were assigned randomly at 16 to 23 weeks of gestation to receive two 2-g doses of metronidazole or placebo 48 hours apart and were re-evaluated for changes in Gram stain score on one occasion ≥2 weeks later. Results: Of 658 women who underwent metronidazole therapy, treatment was successful (score, <7) in 78% of those women who were seen at 2 to 3.9 weeks of gestation, which was similar to women seen ≥10 weeks after treatment. In 683 women who underwent placebo therapy, spontaneous resolution increased significantly from 13% at 2 to 3.9 weeks of gestation to 36% at ≥ 10 weeks of gestation. Spontaneous resolution was more common with lower vaginal pH or lactobacilli on Gram stain at the time of random assignment. Conclusion: The effectiveness of metronidazole therapy of bacterial vaginosis persists for ≥ 10 weeks. Women who underwent placebo therapy had significant remission of bacterial vaginosis over ≥ 10 weeks. Remission was more common when the initial vaginal microbiologic disturbances were less severe. © 2004 Elsevier Inc. All rights reserved.