Background: Suboptimal adherence to adjuvant endocrine treatment (AET) is an important clinical concern. A correlation between CYP2D6 activity and tamoxifen discontinuation has been suggested. The main aim of this study was to investigate the consistency between prescription refill data and reports from medical records on adherence to AET in early breast cancer. We also studied if there was an association between menopausal status, CYP2D6 activity, estimated risk for recurrence and adherence to AET. Methods: 1235 pre-and postmenopausal Swedish breast cancer patients operated 2006 – 2014, genotyped for CYP2D6, who initiated adjuvant tamoxifen treatment, were included in the study. Information on AET was retrospectively collected from both medical records and the Swedish Prescribed Drug Registry. Consistency was defined as dispensed doses of AET divided by AET intake documented in medical records. Adherence was calculated for patients with at least 4.5 years of follow up and was defined as Medical Possession Rate (MPR) ≥ 80 %. Subgroup analyses were performed based on menopausal status, recurrence-risk and CYP2D6 activity. Results: In 84% of the patients the consistency of AET between the sources of information was within 80-125%. Consistency < 80% was most frequent in premenopausal/high risk patients and CYP2D6 Poor Metabolizers (PM). Among 899 patients with at least 4.5 years follow up, adherence to tamoxifen was 72% according to prescription refill data, compared to 77% as reported by medical records. When including aromatase inhibitors adherence increased to 82% and 88%. Adherence did not differ by menopausal status or risk for recurrence. CYP2D6 PM had poorer adherence (54%) to tamoxifen compared to patients with the highest CYP2D6 activity (83%). Conclusions: Consistency between medical records and dispensing data on adherence was better than anticipated. Adherence to AET was good, 82% when including switch to aromatase inhibitors. Surprisingly, CYP2D6 PMs had low adherence to tamoxifen, despite a reduced risk of side effects according to previous studies. Further work is needed to clarify the impact of CYP2D6 activity on adherence to tamoxifen.