Abstract Background Chemotherapy treatments with anthracyclines and Trastuzumab are widely used in different hematologic or solid malignancies but myocardial injury and left ventricular dysfunction, usually called cardiotoxicity (CTX), is a frequent adverse effect. Some studies have demonstrated that early detection and treatment of CTX is essential for the recovery of left ventricular ejection fraction (LVEF), however its behavior after starting treatment is not well established. Purpose Evaluate the behavior of LVEF after starting treatment for heart failure in patients with diagnosis of CTX. Methods Patients with diagnosis of solid or hematologic malignancies scheduled to receive therapy including anthracyclines and/or trastuzumab were prospectively included. Echocardiogram was performed before starting treatment, and then 3 and 6 months after. CTX was defined, according to guidelines, as a drop of LVEF ≥10% to a value <53% and in these patients treatment of left ventricular dysfunction with B blockers and Enalapril/Losartan was promptly initiated and up-titrated to the maximal tolerated dose. Myocardial injury was defined as a drop of LVEF ≥10% and a value ≥53%, with a watchful waiting strategie, considering these patients as a control group. LVEF was re-evaluated 6 months after starting treatment or after diagnosis of myocardial injury. Primary endpoint was defined as a recovery of LVEF ≥10%. Results Of a total of 141 patients of the entire cohort (mean age 56±15 years, 85,8% women and 61,7% with diagnosis of breast cancer),17 patients (12%) met criteria of CTX and 21 (14,9%) of myocardial injury. There were no differences in relation to clinical characteristics or chemotherapy schemes between groups. In patients with CTX (intervention group), 88,2% reached the primary endpoint of recovery of LVEF ≥10% while only 23,8% did it in the control group (p<0,001). Patients with CTX had lowest LVEF at baseline, however no significant differences were observed at 12 months of follow up. Figure 1 represents the behavior of LVEF at baseline, 3, 6 and 12 months. A paired before-after analysis was performed between LVEF at baseline and at 12 months. Patients who received treatment showed no differences in LVEF (63,3±7,2% vs 61,5±9,6%, p=0,5). In contrast, patients who did not receive treatment showed a significant drop in LVEF (72,7±4,4% vs 66,2±5,4%; p<0,001) (figure 2). Conclusion Treatment of left ventricular dysfunction in patients with CTX is associated with a greater recovery of LVEF. Funding Acknowledgement Type of funding sources: None.