A case of a 36-year old woman with HER2-positive early breast cancer treated with adjuvant trastuzumab left ventricle dysfunction and cardiac arrest in ventricular fibrillation mechanism is presented. After having been successfully resuscitated, trastuzumab therapy was withheld, pharmacotherapy (beta-blocker, ACE-I) implemented and ICD was implanted. Echocardiography performed 6 months later, revealed normal systolic function of the left ventricle. The patient died despite further oncologic treatment due to progression of the disease. The authors discuss the approach to this dramatic but lone cardiac side effect of trastuzumab treatment.