--- - |2+ Abstract Background Failing to take endocrine therapy (ET) as prescribed (nonadherence) increases risk of morbidity and mortality from breast cancer recurrence. We explored predictors of nonadherence, including demographic, clinical, treatment, and personal factors, among women newly prescribed ET for early stage breast cancer. We also examined predictors of their thoughts about stopping treatment (TST). Methods A baseline survey prior to ET assessed demographics, illness beliefs, beliefs about medicines, fear of recurrence, symptoms, and negative affect. A follow‐up survey at 3 months repeated these measures with additional questions about nonadherence and TST. Nonadherence and TST were analyzed using logistic and multiple regression, respectively. Patient record review provided clinical data. The baseline survey was completed by 125 women, with a 96% retention rate at follow‐up. Results Thirty‐six percent reported nonadherence, and 30% reported TST. Results of regression analyses showed that TST was most strongly associated with symptom severity at follow‐up, whereas, lower coherence beliefs, and the absence of comorbid conditions were the strongest predictors of actual nonadherence. Conclusion This is the first longitudinal study to examine concurrently the association of demographic, personal and treatment factors with nonadherence, and TST. Findings have potentially important clinical implications; interventions to improve adherence and reduce TST may need to target women's understanding of their diagnosis and treatment, illness beliefs, and symptoms prior to starting therapy. - Psycho-Oncology, Volume 27, Issue 9, Page 2096-2103, September 2018.