Men who have sex with men (MSM) account for the largest proportion of new HIV infections in the United States. Alcohol may facilitate HIV transmission by increasing unprotected anal sex, but few studies have focused on transmission behaviors in HIV-positive MSM. This study explored daily associations between alcohol use and sexual behavior among heavy drinking HIV-positive MSM using a 30-day Timeline Followback interview. Results of generalized estimating equations indicated that greater alcohol consumption on a given day was associated with a linear increase in the odds of having unprotected anal sex with partners of any HIV status. However, the odds of reporting unprotected anal sex with HIV-negative or HIV-status unknown partners increased in a curvilinear fashion, occurring primarily at very heavy levels of use (12+ drinks). Results suggest that very heavy drinking increases the risk of engaging in sexual behavior that has the potential for transmitting HIV to other men.