Objectives: Online HIV prevention studies have been limited in their ability to obtain biological specimens to measure study outcomes. We describe factors associated with willingness of men who have sex with men (MSM) to take a free home HIV test as part of an online HIV prevention study. METHODS: Between March and April 2009, we interviewed 6163 HIV-negative MSM and assessed the willingness to test for HIV infection using a home collection kit. RESULTS: Men reported being very likely (3833; 62%) or likely (1236; 20%) to accept a home HIV test as part of an online HIV prevention study. The odds of being willing to home test were higher for men who were offered incentives of $10 or $25, were black, had unprotected anal intercourse in the past 12 months, and were unaware of their HIV status. Conclusions: Home testing offered as part of online HIV prevention research is acceptable overall and in important subgroups of high-risk MSM.