PurposeExtending adjuvant endocrine therapy (ET) beyond the standard 5 years offers added protection against late breast cancer recurrences in women with early-stage hormone receptor-positive (HR +) breast cancer. Little is known about treatment persistence to extended ET (EET) and the role that genomic assays may play. In this study, we evaluated persistence to EET in women who had Breast Cancer Index (BCI) testing.MethodsWomen with stage I-III HR + breast cancer who had BCI testing after at least 3.5 years of adjuvant ET and ≥ 7 years of follow-up after diagnosis were included (n= 240). Data on medication persistence was based on prescriptions in the electronic health record.ResultsBCI predicted 146 (61%) patients to have low – BCI (H/I)-low – and 94 (39%) patients to have high likelihood of benefit from EET (BCI (H/I)-high). Continuation of ET after BCI occurred in 76 (81%) (H/I)-high and 39 (27%) (H/I)-low patients. Non-persistence rates were 19% in the (H/I)-high and 38% in the (H/I)-low group. The most common reason for non-persistence was intolerable side effects. Patients on EET underwent more DXA bone density scans than those who stopped ET at 5 years (mean 2.09 versus 1.27;p< 0.001). At a median follow-up of 10 years from diagnosis, there were 6 metastatic recurrences.ConclusionsIn patients who continued ET after BCI testing, the rates of persistence to EET were high, particularly in patients with predicted high likelihood of benefit from EET. Use of EET is associated with increased use of DXA scans.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12885-023-11104-w.