To increase understanding of the HIV epidemic among MSM in Barcelona, anonymous questionnaires were completed by 640 MSM recruited in the city in 2002. The prevalence of unprotected anal intercourse (UAI) with casual male partners in the prior 12 months was higher among self-reported HIV-positive men (confirmed through saliva testing) than among men who were HIV-negative or of unknown serostatus (35% vs. 20%, p < .01). The prevalence of UAI with steady male partners was substantially lower among HIV-positive men than other men (28% vs. 60%, p < .01). In multivariate analyses, UAI with casual partners was more likely among HIV-positive individuals; those who used drugs before sex; perceived less acceptance of their sexual orientation by family, friends, or coworkers; and were less concerned about HIV prevention because of antiretroviral therapy (ART). UAI with steady partners was more likely among HIV-negative men with seroconcordant partners, those living with a partner, and men less concerned about HIV prevention because of ART. Findings indicate a need for prevention programs targeting HIV-positive MSM in Barcelona. Attention to substance use and attitudes about HIV prevention are needed for MSM in general.