Anthracyclines are antineoplastic agents that are effective against solid tumors and hematological malignancies. However, drug-induced cardiotoxicity imposes dose limitations. Myocardial damage due to anthracyclines has been assessed by measuring left ventricular ejection fraction (LVEF) or fraction shortening (FS) by echocardiography and criteria for discontinuing treatment have been established based on these indexes. However, cardiotoxicity is already irreversible when either LVEF or FS fulfills these criteria. The Tei-index has recently been established to assess combined systolic and diastolic myocardial function during echocardiography. It can also detect small changes in cardiac function. We therefore surmised that the Tei-index would reflect early myocardial damage induced by anthracyclines. We treated 23 patients with the anthracycline, doxorubicin (DXR), and examined them at least twice during the treatment. An additional dose of DXR significantly correlated with a change in the Tei-index (DeltaTei-index). In contrast, a change in LVEF did not correlate with increased doses of DXR. The DeltaTei-index did not correlate with either LVEF or the Tei-index before treatment. These results suggested that the DeltaTei-index is a more sensitive indicator of early cardiotoxicity induced by anthracyclines than LVEF regardless of its value before treatment.