The availability of improved HIV treatments may prompt reduced concern about HIV and sexual risk. Gay and bisexual men (N = 554, 17% HIV-positive) completed measures of treatment attitudes, sexual risk, and assumptions regarding the infectiousness of sexual partners. A substantial minority reported reduced HIV concern related to treatment advances. Reduced HIV concern was an independent predictor of sexual risk, particularly among HIV-positive men. In response to hypothetical scenarios describing sex with an HIV-positive partner, participants rated the risk of unprotected sex to be lower if the partner was taking combination treatments and had an undetectable viral load, relative to scenarios with a seropositive partner not taking combination treatments. Prevention efforts must address attitudinal shifts prompted by recent treatment successes, stressing the continued importance of safer sex, and that an undetectable viral load does not eliminate infection risks.