Objective To assess the potential clinical signiifcance of myocardial composite index (MCI) in the evaluation of left ventricular myocardial function in breast cancer patients with postoperative chemotherapy by speckle tracking echocardiography (STE). Methods A total of 33 consecutive breast cancer patients with postoperative chemotherapy were prospectively enrolled in Shanghai East Hospital, Tongji University, from February 2012 to February 2013. Thirty-six female healthy volunteers were enrolled as the controls. Routine echocardiographic parameters [left ventricular end diastolic volume (EDV), end systolic volume (ESV), interventricular septum thickness at diastole (IVSTd), left ventricular posterior wall thickness (PWTd) and left ventricular ejection fraction (LVEF)] were obtained. EDV and ESV were corrected for body surface area, and then end diastolic volume index (EDVI) and end-systolic volume index (ESVI) were calculated. STE-derived parameters [left ventricular global longitudinal strain (GLS), global radial strain (GRS), left ventricular twist (LVtw) and myocardial composite index (MCI)] were evaluated. All above data were obtained after 5 anthracycline-based chemotherapeutic cycles and compared with baseline measurements in controls respectively. Receiver operating characteristic (ROC) curves were constructed to determine optimal sensitivity and specificity of continuous variables for the prediction of future cardiotoxicity. Results (1)Changes in EDV, ESV, IVSTd, PWTd and LVEF showed no significant decreases compared with baseline and controls (F=1.35, 2.04, 1.28, 2.34, 2.16, P > 0.05 for all), while the changes in ESVI and EDVI showed statistically decreases (F=4.73, 6.86, P < 0.05 for all);(2)The changes in EDVI and ESVI showed signiifcant decreases compared with baseline (t=6.10, 5.13, P<0.05 for all) and controls (t=5.26, 4.24, P<0.05 respectively);(3)The change in GRS showed no statistical signiifcance compared with baseline and controls (F=1.23, P>0.05), while change in GLS, LVtw and MCI showed statistical significances compared with baseline and controls (F=4.29, 5.02, 12.35, P<0.05 for all);(4)The changes in GLS, LVtw and MCI showed signiifcant decreases compared with baseline (t=5.33, 13.94, P<0.05 respectively) and controls (t=4.97, 4.46, 7.18, P<0.05 all);(5)The area under receiver operating characteristics curve (AUC) of MCI was 0.894, and its optimal cut-off value was-265.7 (%×degree) with sensitivity of 82.6%and specificity of 78.7%. Conclusion MCI based on STE, combining myocardial strain and cardiac twist, may be an accurate predictor in identifying potential cardiotoxicity in postoperative chemotherapeutic survivors with breast cancer.