Background: Low rates of adherence to adjuvant endocrine therapy (AET) is a significant clinical problem. Symptom burden is a key barrier to adherence, but less is known about how changes in symptom burden affects adherence. Moreover, whether higher symptom burden in racial minorities explains their lower rates of adherence, has been relatively unexamined. We used longitudinal data to address these gaps in knowledge. Methods: Using electronic medical records linked with patient-reported data, Medicare and Medicaid claims, we identified women with early-stage, hormone receptor-positive breast cancer who initiated AET from a large cancer center in 08/2007-12/2015, had continuous insurance coverage and ≥1 symptom report before and during AET. Patient-reported symptoms were collected using a tablet-based platform [ConcertAI]. A total of 49 physical symptoms and 11 mental health symptoms were evaluated and classified into 7 physical and 2 mental clusters based on previous literature and clinical expertise. For each cluster, we counted the number of symptoms with moderate severity (≥3 points) at baseline, and with ≥3-point increase during 1-year follow-up. Adherence was defined as the percent of days covered by AET during the 1-year follow-up. We compared Black and White patients’ symptoms at baseline and changes during the therapy, and conducted multivariable regression for patients’ adherence adjusting for race, symptom measures, sociodemographic and clinical characteristics. Results: Black women (n = 168) were diagnosed at a younger age, with more advanced stage, and lived in areas with lower socioeconomic status than White women (n = 391, p<.05). Adherence to AET in the first year was 78.8% for Black and 82.3% for White women (p=.16). Black women experienced higher severity in most symptom clusters at baseline and during the follow-up than White women (p<.05). Neuropsychologic, Vasomotor, Musculoskeletal, Cardiorespiratory, Distress and Despair symptom clusters both at baseline and their increases during the follow-up were associated with 1.2 to 2.6 percentage points (ppt, p<.05) decreases in adherence. This means, each additional count of moderate severity symptoms in these clusters would decrease 4 to 9 days on AET. After adjusting for distress and despair symptoms, Black women had a 6.5 ppt higher adherence rate than White (p<.05). Conclusions: Black women had higher symptom burden at baseline and during the first year of AET. Both physical and mental health symptoms at baseline and the changes during therapy were associated with lower adherence. However, Black women had non-significantly lower rates of adherence despite the higher symptom burden due to unmeasured factors that offset the impact of symptom severity. Better symptom management could improve AET adherence and potentially reduce racial disparities in cancer outcomes.