Objective: The objective of the current work was to assess the frequency of non-adherence behaviors and potential association with patients' experience with healthcare and beliefs in medicines selfreported by patients with four different chronic conditions. Methods: Patients responded anonymously to a survey comprising five non-adherence behaviors (based on physician and patient input), an assessment of patients' experience with healthcare using the validated Instrument to Evaluate the EXperience of PAtients with Chronic diseases (IEXPAC), and a validated Spanish version of the Beliefs about Medicines Questionnaire (BMQ). Associations of nonadherence behavior were analyzed using logistic regression models. Results: Of 1530 respondents, 53.1% reported !1 non-adherence behavior. Non-adherence rates were 59.8% in diabetes mellitus (DM), 56.0% in rheumatic disease, 55.6% in inflammatory bowel disease, and 42.8% in human immunodeficiency virus (HIV) infection patients (p < .001). IEXPAC and BMQ scores were higher in adherent vs. non-adherent patients. In multivariate analysis, non-adherence behavior was strongly associated with lower overall BMQ, lower BMQ Necessity scores and higher BMQ Concerns scores (p < .001 for all), and with a lower IEXPAC self-management score (p ¼ .007), but not with the overall IEXPAC score. Non-adherence was more frequent in DM patients compared with HIV infection patients (p < .001). Conclusions: Patients' beliefs in medicines-a lower perception for the necessity of medication, and higher concerns in taking medication-and low patient self-management experience score were associated with non-adherence behavior. These are modifiable aspects that need to be addressed to increase medication adherence in chronic disease.