BACKGROUND: Young people who engage in substance use are at risk for becoming infected with HIV and diseases with similar transmission dynamics. Effective disease prevention programs delivered by prevention specialists exist but are rarely provided in systems of care due to staffing/resource constraints and operational barriers-and are thus of limited reach. Web-based prevention interventions could possibly offer an effective alternative to prevention specialist-delivered interventions and may enable widespread, cost-effective access to evidence-based prevention programming. Previous research has shown the HIV/disease prevention program within the Web-based therapeutic education system (TES) to be an effective adjunct to a prevention specialist-delivered intervention. The present study was the first randomized, clinical trial to evaluate the comparative effectiveness of this Web-based intervention as a standalone intervention relative to a traditional, prevention specialist-delivered intervention. METHODS: Adolescents entering outpatient treatment for substance use participated in this multi-site trial. Participants were randomly assigned to either a traditional intervention delivered by a prevention specialist (n=72) or the Web-delivered TES intervention (n=69). Intervention effectiveness was assessed by evaluating changes in participants' knowledge about HIV, hepatitis, and sexually transmitted infections, intentions to engage in safer sex, sex-related risk behavior, self-efficacy to use condoms, and condom use skills. FINDINGS: Participants in the TES intervention achieved significant and comparable increases in HIV/disease-related knowledge, condom use self-efficacy, and condom use skills and comparable decreases in HIV risk behavior relative to participants who received the intervention delivered by a prevention specialist. Participants rated TES as easier to understand. CONCLUSION: This study indicates that TES is as effective as HIV/disease prevention delivered by a prevention specialist. Because technology-based interventions such as TES have high fidelity, are inexpensive and scalable, and can be implemented in a wide variety of settings, they have the potential to greatly increase access to effective prevention programming.