OBJECTIVE: To determine risk factors for incident bacterial vaginosis (BV) in young Thai women. STUDY DESIGN: Prospective data from a cohort of 1522 women aged 18 to 35 years, who were enrolled in a study of hormonal contraception and HIV acquisition, were used to evaluate potential risk factors for BV, as diagnosed by Amsel criteria. RESULTS: The median prevalence of BV from 3 to 24 months of follow-up visits was 2.5%. The BV incidence was 10.0 per 100-woman years. Statistically significant factors in multivariable analysis were sex during menstruation [hazard ratio (HR), 1.80; 95% CI, 1.11-2.92], male partners having sex with other women (HR, 2.3; 95% CI, 1.45-2.98), cigarette smoking (HR, 1.79; 95% CI, 1.08-2.98), and trichomoniasis (HR, 15.68; 95% CI, 4.95-49.68). Intravaginal practices were not associated with incident BV in unadjusted or adjusted analysis. CONCLUSIONS: This study supports the association between sexual behaviors and the incident BV. Failure to detect an association between intravaginal practices and incident BV warrants further studies in high-risk populations or in women with a higher prevalence of intravaginal practices. © 2008, American Sexually Transmitted Diseases Association.