There is a pressing need to find an efficacious HIV vaccine and a concomitant need for the recruitment of participants in efficacy trials. These efforts are hampered, however, by a gap between what respondents say they will do regarding research participation, and whether they actually enroll. The current paper examines the size of this gap and proposes psychological reasons for it. Some reasons include the temporal stability of the intention, the time taken to consider its ramifications and plans to deal with them, and the social forces that affect the intention. From this analysis, recommendations are offered to improve recruitment efforts and the predictive power of expressions of willingness to participate.