OBJECTIVES. The purpose of this study was to assess the willingness of intravenous drug users to participate in a preventive human immunodeficiency virus (HIV) vaccine efficacy trial. METHODS. Of the 347 intravenous drug users in methadone treatment who were approached for participation, 257 completed a battery of self-administered questionnaires assessing risk behaviors, interest in vaccine trials, and other vaccine-related information. Data from 16 known seropositives and 1 inconsistent responder were dropped from analyses (n = 240). RESULTS. Fifty-two percent of the subjects expressed a willingness to be one of the first individuals to participate in a preventive HIV vaccine efficacy trial. Subjects who had recently shared needles or works and subjects who trusted the government to ensure vaccine safety were both twice as likely to report interest in participation. Twenty-two percent of subjects reported that they would increase needle sharing if vaccinated. Thirty percent did not know what a vaccine was. CONCLUSIONS. These findings suggest that some in-treatment intravenous drug users would volunteer for a preventive HIV vaccine efficacy trial. Education and counseling will be required to ensure that subjects fully understand the trial's purposes, methods, risks and benefits.