Adherence continues to be a major barrier to successful treatment with highly active antiretroviral therapy (HAART) for HIV-infected individuals. HIV-infected adolescents and young adults face a lifetime of treatment with HAART. Often, individuals who struggle with adherence to HAART face multiple barriers that would therefore impact on the success of any single modality intervention. Thus, we conducted a cross-sectional, observational study to determine the prevalence of personal barriers to adherence and to identify associations between these barriers in HIV-infected subjects, 12 to 24. We studied the following personal barriers to adherence: mental health barriers, high/low self-efficacy and outcome expectancy, and the presence of specific structural barriers. There were 396 subjects infected after age 9 recruited from sites from the Adolescent Trials Network for HIV/AIDS Interventions or the Pediatric AIDS Clinical Trials Group. Of the 396 subjects, 148 (37.4%) self-identified as nonadherent. No significant differences were found between adherent and nonadherent subjects for the presence of mental health disorders. Adherence was significantly associated with all but one structural barrier. Both self-efficacy and outcome expectancy were higher among adherent versus nonadherent subjects (p < 0.0001). Grouping subjects according to low self-efficacy and outcome expectancy for adherence, adherence differed according to the presence or absence of mental health disorders and structural barriers (p < 0.0001). Our data suggest that adolescents have significant rates of non-adherence and face multiple personal barriers. Adherence interventions must address multiple barriers to have the maximum chance for positive effects.