Bacterial vaginosis (BV) may be common among women who report having sex with women (WSW) and frequently occurs in both members of monogamous couples. The results of Gram staining of a vaginal smear were consistent with BV in 81 (25%) and intermediate in 37 (11%) of 326 WSW included in this study. Lactobacilli were detected in 64% of subjects, and 42% of subjects had H2O2-producing strains. BV was associated with a higher lifetime number of female sex partners, failure to always clean an insertive sex toy before use, and oral-anal sex with female partners. Neither recent douching nor sexual practices with male partners were associated with BV. Vaginal smears from each partner were concordant in 55 (95%) of 58 monogamous couples; BV was present in both partners in 16 couples (28%) (P < .001, compared with expected distribution). BV was common among subjects who did not douche, who did not have concurrent sex with male partners, or who did not have a new sex partner, which suggests that other risk factors for BV exist. These data support the hypothesis that sexual exchange of vaginal secretions is a possible mechanism for acquisition of BV.