Anthracyclines are active drugs against breast cancer, but can exert cardiotoxic effects. We analyzed the association between the kinetics of various biomarkers during chemotherapy, and the risk of subsequent cardiac toxicity. 50 patients (49 women) with early breast cancer surgically treated and eligible to anthracycline-based adjuvant chemotherapy were analyzed. The left ventricular ejection fraction (LVEF) together with the plasma concentration of several blood markers was measured at the beginning of anthracycline chemotherapy (t (0)), 5 months (t (1)), 16 months (t (2)), 28 months (t (3)), and 40 months later (t (4)). A single measured LVEF value less than 50% or a clinically overt congestive heart failure (CHF) was considered cardiotoxic effects. We tested whether the kinetics of LVEF and blood biomarkers measured during chemotherapy was predictive of subsequent cardiotoxicity and overall cardiac fitness. The left ventricular ejection fraction measured at the end of treatment as well as the rate of change of hemoglobin concentration during anthracycline-based chemotherapy predicted cardiotoxicity in a 3-year follow-up period. When LVEF at the end of chemotherapy was lower than 53% or hemoglobin blood concentration declined more than 0.33 g/dL/month during chemotherapy, the odds ratio of subsequent cardiotoxicity was 37.3 and 18, respectively. The specificity of these two tests was 93.3% and 80%, whereas the sensitivity was 90.9 and 81.2%, respectively. Testing the rate of change of hemoglobin concentration during anthracycline-based chemotherapy, as well as the left ventricular ejection fraction at the end of treatment, seems a powerful method to assess the effects of anthracyclines on cardiac fitness and identify patients at high risk of CHF. Further validation of these tests on a large cohort of patients and cost-benefit analysis should be encouraged.