OBJECTIVES: to review prevalence and risk factors of trastuzumab's cardiotoxic effects in breast cancer patients METHODS: retrospective chart review of 20 cases recieved trastuzumab for breast cancer treatment at Phramongkutklao hospital during 2010-2011. RESULTS: Average age of patients in this study was 58.2 +/- 9.3 years old. The most common stage of breast cancer were stage III (11/20, 55%).  Average cycle per patients was 21.3 +/- 10.1. There were 12 patients (60%) completed 1 year course of treatment, 7 patients still recieving the treatment and 1 patient had to discontinue treatment before completion.  There were 13 patients (65%) had declining in percentage of left ventricular ejection fraction after trastuzumab treatment, 6 (46%) patients had less than 10% declining of left ventricular ejection fraction, equal to number of patients with 11-20% declining of left ventricular ejection fraction (6 out of 12, 50%). In this study, we found 1 patients who had to discontinue treatment due to over 20% declining of ejection fraction. Due to the small number of cases, we did not found any statistical significant for the risk factors associated with the declining in ejection fraction among patients recieved trastuzumab treatment in this study. CONCLUSIONS: Cardiotoxicy in term of declining in percentage of left ventricular ejection fraction was not uncommon among breast cancer treated with trastuzumab. Closely monitoring patients with schedule echocardiogram is warranted for safety use of this drug.