Introduction: Adherence, which is crucial to the success of antiretroviral therapy (HAART), is currently a major challenge in the care of children and adolescents living with HIV/AIDS. Objective: To evaluate the prevalence of nonadherence to HAART using complementary instruments in a cohort of children and adolescents with HIV/AIDS followed in a reference service in Campinas, Brazil. Methods: The level of adherence of 108 patients and caregivers was evaluated by an adapted standardized questionnaire and pharmacy dispensing records (PDR). Non-adherence was defined as a drug intake lower than 95% (on 24-hour or seven-day questionnaires), or as an interval of 38 days or more for pharmacy refills. The association between adherence and clinical, immunological, virological, and psychosocial characteristics was assessed by multivariate analysis. Results: Non-adherence prevalence varied from 11.1% (non-adherent in three instruments), 15.8% (24-hour self-report), 27.8% (seven-day self-report), 45.4% (PDR), and 56.3% (at least one of the outcomes). 24-hour and seven-day self-reports, when compared to PDR, showed low sensitivity (29% and 43%, respectively) but high specificity (95% and 85%, respectively). In multivariate analysis, medication intolerance, difficulty of administration by caregiver, HAART intake by the patient, lower socioeconomical class, lack of virological control, missed appointments in the past six months, and lack of religious practice by caregiver were significantly associated with non-adherence. Conclusion: A high prevalence of HAART non-adherence was observed in the study population, and PDR was the most sensitive of the tested instruments. 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