Objective: To describe the frequency of human immunodeficiency virus (HIV)/sexually transmitted diseases (STDs) sexual risk behaviors in lesbian and bisexual women. Design: Cross-sectional anonymous self-administered questionnaire. Setting: Women's cultural events; HIV/STD service organizations; women's health collectives. Participants: 504 self-identified lesbian and bisexual women. The sample was composed of predominantly white, well-educated women with a mean age of 35 years. Results: Both lesbian and bisexual women engaged in behaviors that potentially lead to transmission of HIV and other STDs. Lesbians were more likely to have a single sexual partner than were bisexual women. Bisexual women were more likely to report a history of STD. Of those sexually active with male partners, one-third reported multiple male partners in the previous year. Significantly more bisexual women reported condom use. Women with multiple partners were more likely to consider themselves at risk for HIV/STD. Having unprotected sex or male partners was not associated with an increased risk perception. Conclusions: Lesbian and bisexual women are engaging in sexual behaviors that may increase their risk for HIV or STD infection. Culturally sensitive prevention programs are needed.