Background: In hormone-receptor positive (HR+) early breast cancer (EBC) adjuvant endocrine therapy (ET) significantly reduces recurrence and mortality. The most common way to estimate therapy adherence is the use of patient reported outcome measures. Yet, this method is inaccurate due to the social-desirability bias. We therefore aimed to analyze adherence to ET over the first 5 years of treatment by using claims data from a large German health insurance provider (AOK Baden-Wuerttemberg). Methods: Female patients diagnosed with HR+ EBC between July 1^st 2010 and December 31^st 2019 were included into the analysis. Adherence to ET was defined as the ratio of the summed number of pills of filled prescriptions for ET (either aromatase inhibitors or tamoxifen) and the duration of endocrine therapy in days (from the start of the first prescription after completion of surgery and chemotherapy, if applicable). ET administration was observed over 5 years. High/low adherence was defined as a ratio between 0.8 – 1 and 0 – 0.8, respectively. ET duration was divided into yearly intervals and adherence was calculated as above. Continuous high/low adherence was then defined as an adherence between 0.8 – 1 and 0 – 0.8 in each year. Distant recurrence-free survival (DRFS) was calculated from the day of the first diagnosis of EBC until onset of distant metastatic disease. Overall survival (OS) was defined as the period between first diagnosis of EBC until death. Results: In total, 20998 patients were included. 17006 patients (81%) were HR+. Regarding overall adherence during the first 5 years, average adherence was 0.81 ± 0.26, with 72% of all HR+ patients showing high adherence to ET. Of 17006 HR+ EBC patients, 8363 (49%) showed a continuous high adherence to ET and 1731 (10%) showed a continuous low adherence to ET over 5 consecutive years. Continuous adherence to endocrine therapy had a significant impact on DRFS (HR: 0.61; 95% CI: 0.52 - 0.71; p < 0.0001) and OS (HR: 0.52; 95% CI: 0.46 -0.59; p < 0.0001). The 10-year OS rate was 79% / 64% for patients with continuous high / low adherence to endocrine therapy. Conclusions: As measured by the number of prescribed pills, adherence during the first 5 years of endocrine therapy was higher than expected by literature. Continuous adherence to ET over 5 years has a significant impact on OS and DRFS. Future research should focus on identifying factors that influence therapy adherence in HR+ EBC patients.