The objective of this study was to describe the adherence to antiretroviral therapy (ART) among adolescents followed-up in Rio de Janeiro. This cross-sectional study included all adolescents (aged 10-19 years) followed at Instituto de Puericultura e Pediatria Martagão Gesteira and Hospital Universitário Clementino Fraga Filho. Adherence was determined by self-report (number of missed ART doses in three days prior to the interview). Adherence was categorized as taking > or = 95% of the ARTs (adherent), or < 95% (non-adherent). Variables related to demographics and treatment were evaluated and if P value < or = 0.15, they were selected for a logistic regression analysis. One hundred and one adolescents were interviewed. The mean time on ART was 91 months and the mean adherence was 94% of this, 21 were non-adherent, and 80 adherent. The risk factors associated with non-adherence were: if the patient was not concerned about ART, odds ratio (OR) = 3.47 (95% confidence interval [CI] = 1.13-10.68); if they do not carry an extra dose of ART, OR = 6.63 (95% CI = 1.73-25.47); if a health-care worker taught them how to take ART, OR = 0.27 (95% CI = 0.08-0.93). Adherence among adolescents was higher than expected. Factors associated with lack of adherence were: interviewees being unaware of ARTs and lack of commitment to the treatment. Interventions involving these factors must be evaluated.