Background: Symptom burden may contribute to racial differences in cancer treatment adherence and survival. Evidence on changes in symptom burden during chemotherapy and whether these differ by race is scarce. We used patient reported outcomes data collected before and after breast cancer chemotherapy initiation to compare symptom burden by race. Methods: Using electronic medical records of a large cancer center in the southern region of the US, we identified Black and White women diagnosed with stage I-III, hormone-receptor positive breast cancer from January 2007 to December 2015. A tablet-based platform [ConcertAI] was used to collect patient reported symptoms at the point of care. We included patients with at least one completed symptom report before and during chemotherapy. We focused on two standardized composite scores – physical symptoms and treatment side-effect (mean of 50 and standard deviation of 10), and calculated changes in symptoms using the closest report before chemotherapy and the most severe score reported during chemotherapy. Patients with a 10-point increase were classified as having a clinically meaningful increase in symptom burden. We used Oaxaca-Blinder decomposition to quantify racial differences in symptom burden change explained by baseline characteristics. These included baseline symptom scores, sociodemographic characteristics (age, regional level household income and education, state) and clinical characteristics (cancer stage and primary chemo regimen). Results: Among 1,167 included patients, Black women (30%) were younger (52 vs. 55 years old, p<.001), more likely to live in areas with lower median household income and less education, and reported most severe scores about 2 weeks later than White women (p<.05). They were also more likely to report a 10-point increase in symptom burden for physical (68.5% vs. 61.2%, p=.017) and side-effects symptoms score (49.0% vs. 41.4%, p=.015). This was driven by larger increases in selected individual symptoms among Black women, such as sweating, itching, and numbness (under physical symptom score), and hair loss and taste change (under side-effect score). Decomposition analyses showed that baseline characteristics (especially primary chemo regimen) explained 79.2% (p=.002) and 35.2% (p=.131) of the increased probability of Black women reporting a 10-point increase in physical symptom and side-effects scores respectively. Conclusions: Black women with early-stage breast cancer were more likely to report a clinically meaningful increase in treatment side-effects and physical symptoms during chemotherapy compared to White women. Differences by race in physical symptoms scores were mostly explained by baseline characteristics. Future studies should examine whether racial differences in symptom burden translate into differences in treatment adherence and mortality.