Influence of adherence to adjuvant endocrine therapy on disease-free and overall survival: a population-based study in Catalonia, Spain Abstract Objective To assess adherence to endocrine therapy and its relation to recurrence and mortality in women with early breast cancer. Methods This is a retrospective cohort study in population-based cancer registries in two Catalonian provinces of Spain. We included all cases of invasive stage I–III breast cancer diagnosed from 2007 to 2011 and with follow-up to 2017. Adherence to endocrine therapy was measured by means of prescription refills. Patients were considered non-adherent if they filled less than 80% of their prescriptions. After collecting data from patients’ medical records, we analysed clinical variables and their relation with adherence by means of logistic and Cox regression models. Results The study included 2413 women. Five-year adherence was 84.5%; the greatest risk for non-adherence was in women under 50 years of age, diagnosed with stage III cancer, treated with neoadjuvant therapy, or receiving tamoxifen or sequential treatment. Adverse effects were associated with greater adherence. Non-adherence was significantly and independently associated with recurrence (hazard ratio [HR] 1.71, 95% confidence interval [CI] 1.16–2.51) and all-cause mortality (HR 2.11, 95% CI 1.62–2.74), after adjusting for age and tumour stage. Conclusions Although non-adherence was relatively infrequent in this population-based study, its impact on the risk of recurrence and mortality was considerable. Clinicians should make efforts to ensure therapeutic adherence during clinical follow-up of women with breast cancer.