Background: Adjuvant endocrine therapy is recommended for women with oestrogen receptor-positive breast cancer, but many women do not take the medication as directed and they stop treatment before completing the standard 5-year duration. Methods: This retrospective cohort study conducted between 1993 and 2008 of all women with incident breast cancer, who are residing in the Tayside region of Scotland, examined adherence to prescribed adjuvant tamoxifen or aromatase inhibitors (AIs). Survival analysis examined the effect of adherence on all-cause mortality, breast cancer death and recurrence, using linked prescribing, cancer registry, clinical cancer audit, hospital discharge and death records. Results: A total of 3361 women with breast cancer were followed for a median 4.47 years (interquartile range (IQR) ¼ 2.04–8.55). The median overall adherence was 90% (IQR ¼ 90–100%), but the annual adherence reduced after a longer period from diagnosis. Low adherence of o80% was associated with poorer survival (hazard ratios ¼ 1.20; 95% confidence interval ¼ 1.03–1.40, P ¼ 0.019). There was no significant difference for low adherence over the treatment period and recurrence, or breast cancer death, but patients with high annual adherence for 5 years had better outcomes than those with 3 or less. Conclusion: Low adherence to all adjuvant endocrine therapy for women with breast cancer, whether tamoxifen or AI, increases the risk of death.