Recent HIV/AIDS trends in the United States suggest a relative increase in HIV infections among women attributed to injection drug use or heterosexual contact.1 Although the biological risk of female-to-female sexually transmitted HIV is unknown, it is thought to be much lower than the risk of transmission between men and women, including instances in which a condom is used.2 However, studies focusing on women who have sex with women (WSW) have shown that some subgroups of WSW exhibit high levels of sexual risk behaviors with men as well as unsafe injection drug use.3,4 Thus, if risk assumptions are based on self-reported or presumed sexual identity, possible risks for HIV infection may be underestimated in some subgroups of WSW. Few studies have estimated the proportion of WSW or characterized their behavior in samples representative of the population as a whole. Here we describe sexual and drug use behaviors associated with HIV and other sexually transmitted diseases (STDs) among WSW who took part in a door-to-door, population-based survey of women aged 18 to 29 years. The survey was conducted between April 1996 and January 1998 among residents of low-income neighborhoods in Northern California. Study methods have been described in detail in a previous article.5 Of 2547 women who completed the study, 2229 (88%) reported sex exclusively with men, 189 (7%) reported sex with both men and women, and 16 (1%) reported sex exclusively with women. Of the 7 HIV-positive women, 4 reported only male partners, 2 reported both male and female partners, and 1 reported only female partners. None of the 16 WSW who reported sex exclusively with women reported any injection drug use. Therefore, analyses of risk were limited to those who reported sex with both men and women and those who reported sex exclusively with men (Table 1 ▶). TABLE 1 —Prevalence of Sexual Behaviors, Injection Drug Use, and STD/HIV Infections: Population-Based Survey, Northern California Compared with women who had sex exclusively with men, women who had sex with both men and women were significantly more likely to report past and recent high-risk sexual behavior, including sex with an HIV-positive man, multiple male sexual partners, sex with a man who has sex with men, sex with an injection drug user, trading of sex for drugs or money, and anal sex. They were also more likely to report past and recent injection drug use, including use of heroin, cocaine, and speed. Finally, they were more likely to have serological markers for both hepatitis B virus (anti-HBc, HbsAG, or both) and hepatitis C virus (anti-HCV). Rates of HIV and other STDs did not significantly differ owing to the small numbers of these infections. The rates of sexual and injection drug risk activities exhibited by women in this population-based survey who reported sex with both men and women place this group at potentially higher risk of HIV and other STDs than women who were exclusively sexual with either men or women. Prevention efforts should avoid assumptions based on reported sexual identity and should acknowledge that women who report sex with both women and men may be at increased risk for HIV and other STDs.