Background: Tamoxifen and aromatase inhibitors (AI) are adjuvant hormone therapy (HT) routinely used for the treatment of early-stage estrogen-positive breast cancer, their use reduces the risk of breast cancer recurrence by about 30%. Despite the survival benefit, the side effects of HT and other factors may affect patients’ adherence. No large observational study has assessed the 5 year HT adherence for breast cancer. We evaluated HT adherence over a five-year course using the commercial health claims data provided by Truven Health Analytics. Methods: Breast cancer patients who received mastectomy or lumpectomy aged 18 years and older between 1999 to 2014 and initiated HT within 1 year since surgery were selected. The outcome variable was HT adherence rate from years 1 to 5. HT adherence was defined as proportion of days covered (PDC) ≥ 80% during a treatment year. HT adherence rate was computed as adherence patients in a given year divided by all HT users who had insurance coverage in that year. Covariates included patient’s age, geographic region, type of health insurance plan, year at cancer surgery, comorbidity, type of HT drugs, and primary cancer treatment such as surgery, chemotherapy, and radiation treatment. Logistic regression was used to evaluate factors associated with HT adherence. Results: 27,790 patients initiated HT within 1 year of breast cancer surgery. The median age was 55 years old and the median time from breast cancer surgery to HT initiation was 114 days. 29.2% of patients used tamoxifen, 28.2% used anastrozole, 8.9% used letrozole, 1.4% used exemestane, and 32.3% used more than one type of HT drugs. Each year, about 60 to 70% patients were adherent to the treatment. The five year adherence rate decreased from 81.4% in the first year to 21.1% in the 5^th year. Patients who were ≥ 50 years old, lived not in the South, with insurance other than HMO, received lumpectomy or mastectomy combined with chemotherapy and radiation therapy, with no comorbidity, used AI, and received surgery in 2003 were more likely to be adherent to HT. Conclusions: The five year HT adherence rates were low among breast cancer patients. Health care providers need to identify ways to improve HT adherence to prevent breast cancer recurrence. [1996 characters]