Introduction: Highly potent in curing breast cancer, the applicability of chemotherapy is, however, limited by its cardiotoxicity, leading to increase of morbidity and mortality. Currently, prediction of cardiotoxicity has a low accuracy by conventional echo methods, but new methods, such as myocardial deformation imaging, might be superior. Methods: Case report: A 48 year old woman, without cardiovascular risk factors, diagnosed with breast cancer HER 2 positive, was monitored by conventional and speckle tracking echocardiography (STE), before chemotherapy, and aft er doxorubicin and cyclophosfamide (cumulative dose of 240 mg/m2 and 2400 mg/m2 , respectively), aft er docetaxel (cumulative dose of 275 mg/ m2), and aft er a single dose of trastuzumab (640 mg). At baseline, echocardiography showed normal left ventricular (LV) structure and function, with an ejection function (EF) of 62%, mild mitral regurgitation (MR) and normal radial, circumferential, and longitudinal strain (LS of -22%), unchanged after doxorubicin and cyclophosphamide. Results: After docetaxel, echo showed a normal EF (61%), with mild MR, but a decrease of LV deformation by 22% to a LS of -17%. After the infusion of trastuzumab, one month after administration of docetaxel, patient developed an episode of acute heart failure. Echocardiography at this stage showed severe reduction of EF (to 29%), moderate MR (regurgitant volume of 47 ml), with mild systolic pulmonary hypertension (of 45 mm Hg), and markedly decreased LV deformation (LS of -10%). Coronary angiography showed normal epicardial coronary arteries. Six months after cessation of chemotherapy, with maximal therapy for heart failure, patient was compensated, in functional class NYHA II; EF improved to 38%, with reduction of MR (regurgitant volume = 19ml) and improved longitudinal LV strain to -16%. Conclusions: We presented a case of cardiotoxicity in which assessment of longitudinal deformation by STE was the only predictor of further heart failure. Implications: Decrease of longitudinal deformation, assessed by STE, might predict subclinical cardiac dysfunction due to cardiotoxicity. Better knowledge of mechanisms and risk factors for cardiac dysfunction related to chemotherapy and a longstanding echocardiographic follow-up are needed to avoid severe cardiotoxicity and to assess the degree of its reversibility.