Background: It is well known that chemotherapeutic angents have cardiotoxic effect and breast cancer patients treated with anthracycline are at risk of left ventricular (LV) dysfunction. It is important to monitor cardiac function after chemotherapy (CTx), but there are no guidelines for proper timing of LV function evaluation in patients without heart failure symptoms. The purpose of this study is to examine practices for LV function monitoring during the course of CTx and to find out proper timing for LV function evaluation. Methods: Breast cancer patients receiving CTx at Seoul and Yeouido St. Mary���s Hospital from 2005 to 2010 and who had undergone echocardiography before and after CTx were identified. Medical records were reviewed and examined chemotherapeutic regimens, cardiac risk factors, LV function from echocardiography. Definition of LV systolic dysfunction is absolute 10% decrease in LV ejection fraction (EF) from baseline or an LVEF���55%. Results: Data from 2005 to 2008 were excluded because baseline LV function evaluation was not properly done and most of follow-up echocardiography was done after development of symptoms. 154 patients received CTx and had a baseline and follow up echocardiography properly from 2009 to 2010. Follow up interval was 5.1짹2.7 months. 23 patients (14.9%) developed LV systolic dysfunction by definition without heart failure symptoms. Baseline LVEF was 63.8짹6.1% and LVEF at the time of detection of LV systolic dysfunction was 49.9짹6.7%. Cumulative dose of doxorubicin was 356.3짹101.6 mg/BSA and LV dysfunction was developed 7.4짹3.3 months after 1st CTx. A ROC curve was drawn (Figure). Cut-off value was 275.4 mg/BSA (Sensitivity 76.2%, Specificity 66.1%). Conclustion: Baseline LV function evaluation is important to detect LV dysfunction during CTx, but it was neglected in the past. Follow up evaluation for detection of subclinical LV systolic dysfunction should be done according to cumulative dose of anthracycline.