The objective of the study was to present recruitment and retention findings for an Internet based HIV prevention trial evaluated using a randomized controlled design among 15-25-year-olds accessing a website on the Internet. We used a combination of automated electronic and personalized approaches to increase and diversify recruitment, verify participant eligibility and increase retention. We posted 3.5 million banner advertisements, 9354 individuals clicked on the advertisement, 8950 completed an eligibility screener and 3298 a baseline survey; we flagged 675 of these as suspicious and enrolled 2623 individuals. Of these, 2082 (79%) completed a follow-up at one-month and 1398 (53%) completed a two-month follow-up. This retention rate is the highest we have seen for an Internet-based HIV-prevention trial. Our procedures can be replicated in other trials. We stress the importance of using a combination of automated and personalized techniques to increase enrollment, verify eligibility and promote retention.