We examined the influence of attitudes concerning HIV transmission, safe sex, and sexual sensation seeking, as well as negotiated risk reduction with primary partners, on the proportion of unprotected sexual partners (%UASP) among men who have sex with men (MSM). Participants were 263 HIV-seropositive and 238 HIV-seronegative MSM in the Multicenter AIDS Cohort Study between 1999 and 2003 who completed a 20-item attitude survey twice. Behavioral data were collected concurrently and 6-12 months after each survey. Among seropositives, decreased HIV concern and increased safer sex fatigue were associated with higher %UASP at 6 and 12 months. Among seronegatives, increased %UASP at 12 months was associated with safer sex fatigue. At 6 months and 12 months, risk reduction agreements were associated with increased %UASP among seronegatives in seroconcordant monogamous relationships, reflecting their abandonment of condoms in such partnerships. We conclude that HIV prevention efforts should target modifiable attitudes (reduced concern about HIV and safer sex fatigue) and increases in sexual risk-taking of MSM, particularly among HIV+ men having sex with serodiscordant partners.