Non-adherence/persistence to adjuvant endocrine therapy can negatively impact survival. Beliefs about medicines are known to affect adherence. This study aims to identify socio-demographic and clinical characteristics associated with medication beliefs among women taking aromatase inhibitors (AIs). Women completed an online survey on beliefs about AI therapy [Beliefs about Medicines Questionnaire (BMQ)], beliefs about breast cancer [Assessment of Survivor Concerns scale (ASC)], and depression [Personal Health Questionnaire depression scale (PHQ-8)]. Socio-demographic and clinical characteristics were collected. Bivariate analyses and linear regression models were performed to investigate relationships between variables. A total of 224 women reported currently taking AI therapy and were included in the analysis. Significantly higher concern beliefs were found among women who had at least mild depression, experienced side effects from AIs, and previously stopped therapy with another AI. Significant correlations were found between concern and necessity beliefs and cancer and health worry. Women age 70 and older displayed less fear of cancer recurrence and health worry, and a trend towards lower necessity and concern beliefs. No differences were found for other variables. In the regression model, greater necessity beliefs were found with increases in the number of current prescription medications (B = 1.06, 95% CI 0.31-1.81, p = 0.006) and shorter duration of current AI therapy (B = -0.65, 95% CI -1.23 to -0.07, p = 0.029), whereas greater concern beliefs were associated with higher depression scores (B = 1.19, 95% CI 0.35-2.03, p = 0.006). Medication necessity and concern beliefs were associated with a definable subset of patients who may be at higher risk for non-persistence.