Adjuvant hormonal therapy for non-metastatic hormone receptor (HR)-positive breast cancer decreases risk of breast cancer recurrence and increases survival. However, some women do not initiate this life-saving treatment. We used a prospective cohort design to investigate factors related to non-initiation of hormonal therapy among women with newly diagnosed, non-metastatic HR-positive breast cancer recruited from three U.S. sites. Serial interviews were conducted at baseline and during treatment to examine sociodemographic factors, tumor characteristics, and treatment decision-making factors. Multivariate modeling assessed associations between variables of interest and hormonal therapy initiation. Of 1,050 breast cancer patients recruited, 725 (69%) had HR-positive breast cancer, of whom 87 (12.0%) based on self-report and 122 (16.8%) based on medical record/pharmacy fill rates did not initiate hormonal therapy. In a multivariable analysis, non-initiation of hormonal therapy, defined by medical record/pharmacy, was associated with having greater negative beliefs about efficacy of treatment (OR 1.42, 95% CI 1.18-1.70). Non-initiation was less likely in those who found the quality of patient/physician communication to be higher (OR 0.96, 95% CI 0.93-0.99), the hormonal therapy treatment decision an easy one to make (OR 0.45, 95% CI 0.23-0.90) or neither easy nor difficult (OR 0.34, 95% CI 0.20-0.58); and had more positive beliefs about hormonal therapy efficacy (OR 0.40, 95% CI 0.34-0.62). Factors influencing non-initiation of adjuvant hormonal therapy are complex and influenced by patient beliefs regarding treatment efficacy and side effects. Educational interventions to women about the benefits of hormonal therapy may decrease negative beliefs and increase hormone therapy initiation.