Background: Overall survival in women with early breast cancer (EBC) (Stage I-III) has made great strides over the past several decades, with survivors more likely to die from causes other than breast cancer. This study investigates the differential effect of obesity on pre-treatment comorbidity risk in younger versus older women with EBC. Methods: This is a retrospective chart review of women with EBC, age 21 and older, seen at a single institution. Differences in 18 common comorbidities were assessed by age ( > = 65 vs < 65) and Body Mass Index/BMI ( > = BMI 30 vs < 30), with relative risks (RR) estimated from log-binomial regression. Results: In a sample of 590 women, 24% were age 65+, 32% non-white, and 39% had obesity. Patients age 65+ were more likely to report 2+ comorbidities compared to age < 65 (RR 1.62, 69% v 43%). Obese patients (BMI > = 30) were more likely to report 2+ comorbidities compared to BMI < 30 (RR: 1.73, 66% v 38%). Women with obesity were more likely to have hyperlipidemia (RR: 1.85, 28% v 15%) as well as diabetes (RR: 2.51, 17% v 7%). The impact of BMI on comorbidity risk (2+ v < 2) differed by age group (interaction p < 0.001). In the < 65 group, obese patients were more likely to have 2+ comorbidities compared to non-obese (RR: 2.12, 62% v 29%, p < 0.0001). The difference was not as large in the 65+ group (RR: 1.28, 81% v 63%, p = 0.02). Conclusions: This study demonstrates increased comorbidity burden, including hyperlipidemia and diabetes, in women with obesity and EBC, especially those < 65. Providers should ensure that patients with obesity and EBC are appropriately screened and treated for underlying comorbidities. Future research should assess the impact of weight loss on overall survival in EBC patients.