BACKGROUND AND AIMS: Heavy drinking is associated with increased risk of incident HIV infection among men who have sex with men (MSM). Past studies suggest that this association may be due to the tendency for intoxication to interfere with condom use. However, research on potential causal mechanisms explaining this relationship has been limited primarily to laboratory studies. In this study, we tested several potential mediators of the relationship between alcohol use level and HIV risk behavior. DESIGN: Ecological momentary assessment (EMA) methods conducted over a 30-day period. SETTING AND PARTICIPANTS/CASES: MSM (n = 100) in the northeastern United States. MEASUREMENTS: Participants completed daily diary surveys and up to six experience sampling surveys randomly prompted throughout the day. FINDINGS: Very heavy levels of drinking (12+ drinks) increased the odds of engaging in any sex [odds ratio (OR) = 1.87, P < 0.001]. Coefficient products and 95% confidence intervals indicated that both subjective sexual arousal (OR = 1.52, P < 0.001) and sex intentions (OR = 1.74, P < 0.001) significantly mediated the association between very heavy drinking and the odds of sex. When participants reported sex, the odds of engaging in high-risk condomless anal sex (CAS) increased incrementally after drinking heavily (five to 11 drinks; OR = 3.27, P = 0.006) and very heavily (12+ drinks; OR = 4.42, P < 0.001). Only subjective sexual arousal significantly mediated the association between alcohol use level and high-risk CAS (OR = 1.16, P = 0.040). CONCLUSIONS: Increases in subjective sexual arousal after drinking heavily appear to partly account for alcohol-related HIV risk behaviors in the daily lives of men who have sex with men. Alcohol's role in strengthening motivationally consistent emotional states may therefore play a more important role in facilitating alcohol-involved HIV risk than explicit sexual motivation.