A behavioral intervention for men who have sex with men (MSM) was created for online delivery. The nine-minute video drama "The Morning After" (www.hivbigdeal.org) was designed to promote critical thinking about HIV risk. MSM were recruited for the evaluation through banner ads on a subscription-based gay sexual meeting web site. Participants viewed the intervention online and completed online behavioral questionnaires at enrollment and three month follow-up. Each participant served as his own control. No participant incentives were provided. Follow-up was completed by 522 (54%) of 971 eligible men. Men completing follow-up were similar to those who did not by age and prevalence of unprotected anal intercourse (UAI) but reported more lifetime sex partners. They also differed somewhat by race, education, and HIV testing. In the three months after the intervention, men were significantly more likely to disclose HIV status to partners (odds ratio [OR] = 3.37, p<0.001) and less likely to report a casual partner or UAI in their most recent sexual encounter than at baseline. Of 120 men HIV tested during follow-up, 17 (14%) reported being HIV seropositive. These findings show that MSM at high risk for HIV will participate in a brief online video intervention designed to engage critical thinking and that significant, self-reported increases in HIV disclosure and decreases in risk behavior occurred three months after MSM viewed the video.