Treatment of breast cancer with trastuzumab is complicated by cardiotoxicity in up to 34% of the patients. In most patients, trastuzumab-induced cardiotoxicity (TIC) is reversible: left ventricular ejection fraction (LVEF) improves after trastuzumab withdrawal and with, or sometimes without, initiation of heart failure (HF) therapy. The reversibility of TIC, however, is not foreseeable, and identification of patients at risk and of those who will not recover from cardiac dysfunction is crucial. The usefulness of troponin I (TNI) in the identification of patients at risk for TIC and in the prediction of LVEF recovery has never been investigated. In total, 251 women were enrolled. TNI was measured before and after each trastuzumab cycle. LVEF was evaluated at baseline, every 3 months during trastuzumab therapy, and every 6 months afterward. In case of TIC, trastuzumab was discontinued, and HF treatment with enalapril and carvedilol was initiated. TIC was defined as LVEF decrease of > 10 units and below 50%. Recovery from TIC was defined as LVEF increase above 50%. TIC occurred in 42 patients (17%) and was more frequent in patients with TNI elevation (TNI+; 62% v 5%; P < .001). Twenty-five patients (60%) recovered from TIC. LVEF recovery occurred less frequently in TNI+ patients (35% v 100%; P < .001). At multivariate analysis, TNI+ was the only independent predictor of TIC (hazard ratio HR, 22.9; 95% CI, 11.6 to 45.5; P < .001) and of lack of LVEF recovery (HR, 2.88; 95% CI,1.78 to 4.65; P < .001). TNI+ identifies trastuzumab-treated patients who are at risk for cardiotoxicity and are unlikely to recover from cardiac dysfunction despite HF therapy.