BACKGROUND: Adherence to treatment, a public health issue, is of particular importance in chronic disease therapies. Primary care practices offer ideal venues for the effective care and management of these conditions. The aim of this study is to assess adherence to treatment and related-factors among patients with chronic conditions in primary care settings. METHODS: A cross-sectional study was conducted among 299 adult patients with ≥1 chronic condition(s) and prescribed medication in primary healthcare centers of Spain. The Morisky-Green-Levine questionnaire was used to assess medication adherence via face-to-face interviews. Crude and adjusted multivariable logistic regression models were used to analyze factors associated with adherence using the Multidimensional Model proposed by the World Health Organization - social and economic, healthcare team and system-related, condition-related, therapy-related, and patient-related factors. RESULTS: The proportion of adherent patients to treatment was 55.5%. Older age (adjusted odds ratio 1.31 per 10-year increment, 95% CI 1.01-1.70), lower number of pharmacies used for medication refills (0.65, 95% CI 0.47-0.90), having received complete treatment information (3.89, 95% CI 2.09-7.21), having adequate knowledge about medication regimen (4.17, 95% CI 2.23-7.80), and self-perception of a good quality of life (2.17, 95% CI 1.18-4.02) were independent factors associated with adherence. CONCLUSIONS: Adherence to treatment for chronic conditions remained low in primary care. Optimal achievement of appropriate levels of adherence through tailored multifaceted interventions will require attention to the multidimensional factors found in this study, particularly those related to patients' education and their information needs.