Procedures must be developed to ensure that valid informed consent is obtained from participants in HIV vaccine efficacy trials. A prototype informed consent process was evaluated among 4,892 persons at high risk for HIV infection in the HIV Network for Prevention Trials Vaccine Preparedness Study (VPS), a prospective cohort study of HIV seroincidence in eight U.S. metropolitan areas. Twenty percent of VPS participants were selected at random to undergo the prototype informed consent process at VPS month 3. Participants' knowledge of 10 key HIV vaccine trial concepts and willingness to participate in HIV vaccine efficacy trials were assessed and compared at baseline and semiannually thereafter for 18 months. Knowledge of HIV vaccine trial concepts was low at baseline. Participation in the prototype process was associated with substantial and sustained increases in knowledge (relative risks for the 10 items, 1.04-2.26), which were of similar magnitude across HIV risk groups, race/ethnicity, and educational levels. It is recommended that the prototype informed consent process be adopted for future HIV vaccine efficacy trials as well as for clinical trials in other research areas.