Sexually active men who have sex with men (MSM) at 5 Seattle clinics were assessed for bacterial sexually transmitted diseases (STDs), human immunodeficiency virus (HIV)-discordant partnerships, sexual behavior, and drug use. Of the HIV-positive men, 45% reported having HIV-negative sex partners and 42% reported having sex partners with unknown serostatus during the past 2 months, whereas 14% and 57% of HIV-negative men reported having HIV-positive and unknown-serostatus sex partners, respectively. Correlates of sex partners with unknown serostatus were recruiting sex partners at bathhouses or parks. Gonorrhea, chlamydia, or syphilis was diagnosed in 12% of HIV-positive and 13% of HIV-negative MSM, and the rates did not differ between men with HIV-concordant and HIV-discordant partnerships. High prevalences of bacterial STDs and HIV-discordant partnerships emphasize the need for interventions to foster serostatus discussion, condom use, fewer anonymous partners, and STD screening.