Rates of adherence and persistence with endocrine therapy regimens (i.e., tamoxifen, aromatase inhibitors) by breast cancer survivors are suboptimal, with negative implications for prognosis. This study identified potential contributors to nonadherence and nonpersistence. From an online breast cancer research registry (Army of Women) including approximately 51,000 breast cancer survivors, we recruited 1,371 women who currently were taking endocrine therapy and 94 nonpersisters (i.e., diagnosed during the prior 5 years and on endocrine therapy within the prior 12 months, but no longer taking it). Participants completed an online questionnaire assessing demographic/medical characteristics, general and cancer-related psychosocial variables (i.e., depressive symptoms, anxiety, patient-oncologist relationship quality, cancer recurrence worry, general symptoms), and endocrine therapy-specific variables (i.e., endocrine therapy-related symptoms, perceived endocrine therapy necessity, long-term therapy use concern, endocrine therapy-related emotions). Two weeks later, current users were re-contacted to complete an endocrine therapy adherence measure. In a final regression model, patient-reported nonadherence among current users was significantly associated with lower financial status, a prior switch in endocrine therapies, a poorer relationship with the oncologist, and lower perceived need for and more negative emotions regarding endocrine therapy (adjusted R (2) = 0.15, P < 0.001). In a final logistic regression model, endocrine therapy nonpersisters were significantly more likely than current users to report depressive symptoms, as well as more negative emotions and lower positive emotions related to endocrine therapy (adjusted R (2) = 0.10, P < 0.001). In addition to demographic/medical variables, several potentially modifiable psychosocial characteristics are likely to contribute to endocrine therapy nonadherence and nonpersistence.