Objective and Goal: Bacterial vaginosis (BV) is extremely common and is associated with adverse obstetrical and gynecological outcomes. The etiology of the microbiologic changes is unknown. The objective of this study was to determine risk factors associated with incident BV. Study: 96 women without BV were followed prospectively for 1 year for the development of BV. Thirty-seven of their male partners were also studied. Results: The incidence rate of BV was 2.33/person-year (95% CI, 1.63-2.50). Median time to development of BV was 81 days. Incident BV was significantly associated with exposure to a new sexual partner (RR, 1.13; 95% CI, 1.02-1.25; P = 0.02) and frequency of vaginal sex since last visit (RR, 1.07; 95% CI, 1.01-1.15; P = 0.03). Use of condoms with occasional partners was protective (RR, 0.80; 95% CI, 0.67-0.98; P = 0.003). In multivariate analysis, sex with a new partner since the prior visit was the only behavior found to be significantly associated with incident BV (RR, 1.74; 95% CI, 1.05-2.87; P = 0.03). Conclusion: These data support the sexual transmission of BV. Copyright © 2005, American Sexually Transmitted Diseases Association. All rights reserved.