We examined whether beliefs about antiretroviral (ART) efficacy and reinfection prospectively predicted subsequent condomless anal intercourse (CAI). Men who have sex with men in San Francisco (N = 773) were recruited for a longitudinal study using time-location sampling. HIV-negative men were more likely to have sero-discordant receptive CAI and HIV-positive men were more likely to have sero-discordant insertive CAI if they previously reported these behaviors at baseline and reported less concern about HIV transmission due to ART. HIV-positive men were more likely to report sero-concordant CAI at follow-up if they reported this behavior at baseline. Previous sexual behavior was consistently the strongest predictor of future sexual behavior. Previous sexual behavior and optimistic beliefs about ART for treatment and prevention predicted subsequent sexual behavior with sero-discordant partners. Since individual-level and population-level benefits of ART depend on persons maintaining adequate drug concentrations, prevention messages should continue emphasizing treatment adherence and practicing a combination of risk-reduction strategies.