BACKGROUND: Nonadherence to antiretroviral therapy (ART) may lead to viral replication and development of antiretroviral resistance. OBJECTIVE: To identify the factors associated with nonadherence to ART among people living with the human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) (PLWHA). DESIGN AND SETTING: Cross-sectional study in a tertiary-level hospital in northeastern Brazil. METHODS: Intake of less than 90% of the antiretroviral drugs prescribed in the last week prior to the interview was defined as nonadherence. Intake was evaluated using a questionnaire. Descriptive and multivariate analyses were conducted on the study population, with estimation of the respective odds ratios and 95% confidence intervals. RESULTS: The prevalence of nonadherence was 28.4%. Significant associations were found regarding the following variables: age less than 35 years, smoking, sedentary lifestyle, lack of medication and lack of knowledge regarding the patient's HIV status, on the part of the patient's partner or family. CONCLUSIONS: Encouragement of adherence to antiretroviral therapy is one of the fundamental pillars of treatment for HIV-infected patients. The high proportion of nonadherence (28.4%) and the predictive factors related to this indicate that it is necessary to improve patients' adherence to antiretroviral therapy.