<p>Despite the proven clinical benefits of use of adjuvant hormone therapy with tamoxifen or aromatase inhibitors for breast cancer, adherence to and persistence with adjuvant hormone therapy are suboptimal. It is critical to understand the clinical and economic impacts of low adherence to and low persistence with adjuvant hormone therapy in breast cancer. The overall objective was to assess associations between adherence to and persistence with adjuvant hormone therapy and mortality, healthcare utilization, and healthcare costs among older women with breast cancer. A retrospective longitudinal analysis of the Surveillance, Epidemiology, and End Results (SEER) registry linked with Medicare claims was conducted. This study included 25,796 older women diagnosed with hormone receptor-positive stage I-III breast cancer from 2009 through 2017. Adherence was defined as having proportion of days covered (PDC) of 0.80 or more. Persistence was defined as having no hormone therapy discontinuation, i.e., a break of at least 180 continuous days. Length of persistence was calculated as time from therapy initiation to discontinuation. All analyses were conducted using SAS 9.4 and RStudio for Linux environment. An <em>a priori</em> alpha level of 0.05 was used to determine significance for all the analyses. Time-dependent Cox models were used to assess associations between adherence to and persistence with adjuvant hormone therapy and mortality. Hurdle generalized linear mixed models were used to assess associations between adherence and persistence with annual number of hospitalizations, hospital days, hospital outpatient visits, inpatient costs, and outpatient costs across five years to account for excess zeroes.  Generalized linear mixed models were used for other types of healthcare utilization and costs. Annual adherence rates were 78.1 percent, 75.2 percent, 72.4 percent, 70.0 percent, and 61.5 percent from year-one to year-five after hormone therapy initiation. Persistence rates were 87.5 percent, 81.7 percent, 77.1 percent, 72.9 percent, and 68.9 percent through cumulative intervals of one year up to five years after hormone therapy initiation. Adherence was associated with lower risk of all-cause mortality, but was not significantly associated with breast cancer-specific mortality. Both being persistent and longer persistence were associated with lower risk of all-cause mortality and lower risk of breast cancer-specific mortality. Being adherent was associated with fewer hospitalizations, fewer hospital days, fewer emergency room visits, and fewer hospital outpatient visits, but was not associated with physician office visits. Being persistent was associated with fewer hospital days, fewer emergency room visits, and fewer hospital outpatient visits, but was associated with more physician office visits. Longer persistence was associated with fewer hospital days, fewer emergency room visits, and fewer hospital outpatient visits, but was not significantly associated with physician office visits. Adherent women had lower inpatient costs, lower outpatient costs, lower medical costs, and lower total healthcare costs despite higher prescription drug costs. Both being persistent and longer persistence were associated with lower inpatient costs, lower outpatient costs, lower medical costs, and lower total healthcare costs despite higher prescription drug costs. </p>