Background. The most important anthracycline side effect is cardiotoxicity, resulting in congestive heart failure (HF). Early detection of cardiac dysfunction and appropriate treatment can improve outcomes and reduce the progression of HF. The aim of our study was to evaluate changes in clinical data, echocardiographic parameters, and NT-proBNP, as well as their associations with early anthracycline-induced cardiotoxicity (AIC) in patients treated with anthracycline-based chemotherapy.Methods and Materials. Patients with breast cancer were prospectively assessed with echocardiography, as well as NT-proBNP testing at baseline, (T0), after two cycles (T1) and four cycles (T2) of chemotherapy. AIC was defined as a new decrease in the LVEF of 10 percentage points, to a value below the lower limit of normal.Results.We evaluated 85 patients aged 54.5 ± 9.3 years. After a cumulative dose of 237.9 mg/m2of doxorubicin, 22 patients (25.9%) met the criteria of AIC after chemotherapy. Patients who subsequently progressed to cardiotoxicity had demonstrated a significantly larger impairment in LV systolic function compared to those who did not develop cardiotoxicity (LVEF: 54.0 ± 1.6% vs. 57.1 ± 1.4% at T1,p< 0.001, and 49.9 ± 2.1% vs. 55.8 ± 1.6% at T2,p< 0.001; GLS: −17.8 ± 0.4% vs. −19.3 ± 0.9% at T1,p< 0.001, and −16.5 ± 11.1% vs. −18.5 ± 0.9% at T2,p< 0.001, respectively). The levels of NT-proBNP increased significantly from 94.8 ± 43.8 ng/L to 154.1 ± 75.6 ng/L,p< 0.001. A relative decrease in GLS ≤ −18.0% (sensitivity: 72.73%; specificity: 92.06%; AUC, 0.94;p< 0.001) and a relative increase in NT-proBNP > 125 ng/L (sensitivity: 90.0%; specificity: 56.9%; AUC, 0.78;p< 0.001) from baseline to T1 predicted subsequent LV cardiotoxicity at T2.Conclusions. Decrease in GLS and elevation in NT-proBNP were significantly associated with AIC, and these could potentially be used to predict subsequent declines in LVEF with anthracycline-based chemotherapy.