We analyzed 127 consecutive patients who received trastuzumab-based chemotherapy from December, 2003 to February, 2009 in our hospital. Of 127 patients, cardiac dysfunction appeared in 6 patients(4. 7%). Cardiac dysfunction was defined as a decline in left ventricular (LV) ejection fraction (EF) < or =55% with absolute reduction of at least 10% from baseline. Among the 6 patients with cardiac dysfunction, one patient suffered symptomatic heart failure. Other patients were asymptomatic. The 4 patients of the 5 patients recovered their cardiac dysfunction after withdrawal of trastuzumab. Patients with trastuzumab-associated cardiac dysfunction had a history of administration of epirubicin or taxane, lower registration LVEF, and larger LV end-diastolic dimension (> or =49 mm). We recommend that LV function be assessed by echocardiography or multigated radionuclide angiography scans prior to instituting trastuzumab therapy and at three-month intervals during therapy. Trastuzumab should be discontinued in patients who develop a decrease in LVEF below 45% or congestive heart failure.