Anti-neoplastic chemotherapy can determine various side effects, including cardiotoxicity, and no real guidelines for its early detection and management have been developed. The aim of this study is to find some plasmatic markers able to identify breast cancer patients that are at greater risk of developing cardiovascular complications during chemotherapy, in particular heart failure. A prospective study on 100 breast cancer patients with mean age of 66 years in adjuvant treatment with anthracyclines, taxanes, and trastuzumab was performed. Patients underwent cardiological examination before starting treatment (T0) and at 3 months (T1), 6 months (T2), and 1 year (T3) after treatment. Evaluation of serum cardiac markers and N-terminal pro-brain natriuretic peptide (NT-proBNP) was performed at T0, T1, T2, and T3, simultaneously to electrocardiogram and echocardiogram, showing a significant increase in NT-proBNP concentration (p > 0.0001) at T1, T2, and T3, before left ventricular ejection fraction decrease became evident. Human epidermal growth factor receptor 2 (HER2)-negative patients were more susceptible to mild hematological cardiotoxicity, while HER2-positive patients were more susceptible to severe cardiotoxicity. A significant correlation between NT-proBNP increased values after chemotherapy and prediction of mortality at 1 year was evidenced. From our experience, serum biomarker detection was able to support an early diagnosis of cardiac damage, also in the absence of left ventricular ejection fraction decrease. Therefore, the evaluation of specific plasmatic markers for cardiac damage is more sensitive than echocardiography in the early diagnosis of chemotherapy-related cardiotoxicity; furthermore, it can also add a prognostic value on outcome.