PURPOSE: To compare prevalence and describe predictors of antiretroviral treatment adherence among adolescents with HIV acquired perinatally (PIY) or through risk behaviors (BIY). METHODS: Data were obtained from the baseline assessment of Adolescent Impact, an intervention for HIV-infected adolescents receiving care in three U.S. cities. Patients self-reported missed medication doses as well as medication factors, HIV knowledge, disclosure, substance use, mental health, and social support through face-to-face or computer-assisted interviews. RESULTS: Of 104 participants, 68 (65.4%) reported full adherence. Compared with BIY, PIY were younger, had greater HIV disease severity, and had more structural supports. Adjusting for transmission mode (PIY vs. BIY), nonadherence by self-report was associated with higher viral load (VL) (adjusted odds ratio [AOR] = 1.5, confidence interval [CI] = 1.03, 2.18). Nonadherent adolescents were significantly likely to have had AIDS, discussed HIV disease with providers, reported difficulty with medication routine, experienced internalizing behavior problems, and used drugs. In multivariate analyses, independent predictors of nonadherence included acquiring HIV behaviorally (AOR = 4.378, CI = 1.055, 18.165), ever having AIDS (AOR = 4.78, CI = 1.31, 17.49), perceiving difficult medication routine (AOR = 1.84, CI = 1.07, 3.16), discussing disease indicators with provider (AOR = 4.57, CI = 1.74, 11.98), and missing doses because of forgetting (AOR = 2.53, CI = 1.29, 4.96). Adjusting for transmission mode, detectable VL was associated with lower recent CD4(+) lymphocyte counts, discussing disease indicators with providers, and missing doses because of forgetting or being depressed. Low recent CD4(+) lymphocyte counts (AOR = .988, p = .024) but fewer HIV symptoms (AOR = .466, p = .032) and missing doses because of forgetting (AOR = 1.76, p = .05) were independently associated with detectable VL in multivariate analysis. CONCLUSIONS: Despite differences between groups, nonadherence was associated with severity of illness, difficult medication routine, and forgetfulness. Beyond individual needs, both groups of adolescents had suboptimal adherence and would benefit from simplified medication routines and organizational skills.