Adjuvant hormonal therapy (HT) for breast cancer improves survival, yet studies have shown that early discontinuation and suboptimal adherence are common. We aimed to expand existing literature by describing patterns of HT use, specifically focusing on the prevalence and predictors of treatment interruptions of varying durations. We identified 2,207 women who were diagnosed with early-stage breast cancer and who initiated adjuvant HT between July 1, 2000 and 2005, and were followed through August 1, 2006, at a New England health plan. Of 58% of women who initiated HT within 12 months after diagnosis, 769 (54.6%) used tamoxifen, 354 (25.1%) used an aromatase inhibitor, and 285 (20.3%) switched between the two agents during the follow-up period. By the end of the first year of treatment, 79% of women remained on therapy without gaps exceeding 60 days and 85% without gaps exceeding 180 days. By year 5, only 27 and 29% remained without 60- and 180-day gaps, respectively. Results from extended Cox proportional hazards regression models indicated that only age ≥ 70 years (vs. less than 50 years) was consistently associated with an increased likelihood of treatment gaps, with hazard ratio (HR) of 2.00 [95% CI 1.36-2.94] for gaps of ≥ 60 days, HR 2.09 [1.38-3.16] for gaps of ≥ 90 days, and HR 2.14 [1.36-3.37] for gaps of ≥ 180 days in the first follow-up year, with similar results in subsequent years. Longer gaps were associated with a lower likelihood of resuming therapy. In summary, interruptions in HT therapy began in the first year after initiating treatment and continued through subsequent years and were common among insured women, particularly the elderly. Clinicians caring for breast cancer survivors should explicitly ask women about their medication use, explore barriers to adherence, and encourage them to continue long-term therapy as advised by treatment guidelines.