To determine whether optimism about highly active antiretroviral therapy is associated with sexual risk behavior among young adult HIV-negative gay and bisexual men, and to test 2 alternative explanations for this association: that treatment optimism leads to increased sexual risk or that treatment optimism is the result of previous sexual risk. Data on sexual risk behavior, treatment optimism, and perceived susceptibility to HIV infection were obtained from a sample of 538 HIV-negative or untested gay and bisexual men (ages 18-30) who were not in monogamous relationships. Follow-up data were collected 18 months later. In the cross-sectional data, treatment optimism was associated with the 2-month cumulative incidence of unprotected anal intercourse (UAI) with nonprimary partners; however, this effect was observed only among men who felt highly susceptible to HIV infection. Longitudinal analyses revealed that treatment optimism did not predict subsequent UAI, but UAI did predict later treatment optimism. Treatment optimism is associated with sexual risk behavior among young adult gay and bisexual men. However, these data suggest that optimism may result from, rather than precede, sexual risk.