Sustained recruitment over time of tens of thousands of clinical trial volunteers is essential to the development of safe and efficacious HIV vaccines. This study explored, in depth, reasons for declining to enroll among persons screened in as eligible for a Phase IIb prophylactic HIV vaccine trial. Thirteen non-enrollees completed a self-administered questionnaire; of those, 11 completed a 1-h follow-up interview. Interviews were transcribed verbatim and themes derived using narrative thematic analysis and NVivo software. Concerns about negative social consequences of false HIV-positive tests, trial uncertainties, side effects, double-blind assignment, trial duration, uncertain efficacy, behavioral disinhibition and stigma emerged as reasons for declining to enroll. Social, psychological and emotional dimensions of HIV vaccine trial participation--including false-positives and anticipated stigma and discrimination, possible impact on intimate relationships, and concerns about behavioral disinhibition--suggest that provision of voluntary trial-related psychosocial counseling, a trial ombudsperson, alternate trial sites, and systematic community engagement in trial planning, recruitment and evaluation may facilitate informed participation in safe and ethically conducted HIV vaccine trials.