BACKGROUND: The National Surgical Adjuvant Breast and Bowel Project B35 and International Breast Cancer Intervention Studies II Ductal Carcinoma In Situ trials showed similar treatment effects of anastrozole and tamoxifen in reducing cancer recurrence risk among ductal carcinoma in situ (DCIS) patients. Studies have shown low levels of hormone therapy drug initiation for DCIS patients, but the current body of literature lacks information on the 5-year adherence rates for these drugs from population-based studies. METHODS: This study evaluated the initiation and 5-year adherence levels for women aged 66 to 85 years who had been diagnosed with estrogen receptor (ER)-positive DCIS between 2007 and 2011 according to the Surveillance, Epidemiology, and End Results and Texas Cancer Registry databases linked to Medicare claims. Chi-square tests, trend tests, and logistic regression were used to identify factors associated with treatment initiation. RESULTS: There were 2871 women with ER-positive DCIS, and approximately 45% began treatment with tamoxifen or aromatase inhibitors (AIs) within 1 year of their DCIS diagnosis. The median age was 73 years for the users and 75 years for the nonusers. Women aged 66 to 70 years who underwent lumpectomy and radiation therapy were significantly more likely to initiate hormone therapy. The initiation of therapy was also significantly associated with patients' geographic location, education, marital status, diagnosis year, and race/ethnicity. Among users, adherence decreased from 67% in the first year to 30% in the fifth year. CONCLUSIONS: Initiation and adherence levels for tamoxifen or AIs among older women with ER-positive DCIS are low. Future studies should develop methods to ensure that informed discussions take place between health care providers and patients regarding hormonal therapy for cancer prevention. (C) 2016 American Cancer Society.