Despite the proven therapeutic benefits, trastuzumab shows cardiotoxic side effects in up to 25% of treated patients, especially if used in combination with doxorubicin. Approximately 5-10% of all patients treated with trastuzumab in the adjuvant setting develop (mostly asymptomatic) cardiac dysfunction and 1-3% develops severe and highly symptomatic heart failure with NYHA functional class III - IV CHIEN 2006. While both early (peaking 4 months post chemotherapy) and late (up to several years post exposure) cardiac toxicities leading to congestive heart failure have been described following administration of anthracycline-based chemotherapy, the pathophysiology of this toxicity, particularly in the late phase remains incompletely understood and several mechanism have been proposed; the interruption of the HER-2 signaling pathway in the heart seems to affect the ability of cardiomyocytes to grow, repair and survive. Studies in HER-2 knock-out mice have shown that a dilated cardiomyopathy develops following cardiac stress with anthracycline administration OZCELIK 2002. A direct toxic effect of the HER-2 blockade on the myocardial tissue level has been discussed, too OZCELIK 2002, trastuzumab seems to block anti-apoptotic signaling pathways, leading to cardiac dysfunction OZCELIK 2002.