Breast cancer is the most common cancer and the most frequent cause of death in women. Doxorubicin, an anthracycline, is an important drug due to its efficacy in treating solid cancers, especially breast cancer. However, this drug is often responsible for cardiotoxicity that may affect more than 25% of patients. This study aimed to evaluate the red cell distribution width (RDW) in women with breast cancer to monitor adverse events associated with the use of doxorubicin. A prospective study of 80 women with breast malignancy undergoing neoadjuvant doxorubicin-based chemotherapy was conducted. The patients were evaluated at baseline (T0), just after the last cycle of chemotherapy with doxorubicin (T1), and 1 year after the treatment (T2). There was a significant increase over the time points for the RDW (p< 0.001). There was a negative correlation between the RDW and C-reactive protein (CRP) levels at T1. The RDW did not show a significant difference between the groups classified according to cardiotoxicity. Based on these results, the RDW is a cost-effective test that shows a relationship with the doxorubicin response, but not with cardiotoxicity. It is a potential biomarker to evaluate patients with breast cancer after they receive chemotherapy with doxorubicin.