Background: As cancer therapies have improved, patient life spans have been extended but quality of life has been threatened by chemotherapy induced cardiotoxicity. Most cardiac complications remain unobserved until specific symptoms develop. Speckle-tracking echocardiography is a sensitive imaging modality in detecting early occult myocardial dysfunction. Methods: A total number of 35 patients newly diagnosed with breast cancer and preparing for epirubicin therapy were prospectively recruited. Echocardiography, including speckle-tracking echocardiography, was performed sequentially at baseline (T1), after the first cycle (T2) and after the third cycle (T3) of epirubicin. At each visit, the severity of dyspnea was evaluated by the assessment scale. Results: Compared with the baseline, right ventricular longitudinal strain (RVLS_FW) at T2 significantly declined (-22.49  4.97 vs. -18.48  4.46, p = 0.001), which was also positively associated with the development of dyspnea (R2 = 0.8, p = 0.01). At T3, both the left ventricular global longitudinal strain and RVLS_FW were significantly impaired (-21.4  4.12 vs. -16.94  6.81%; -22.49  4.97 vs. -16.86  7.27%, p = 0.01; 0.001, respectively). Also, the accumulating dose of epirubicin positively correlated with the development of dyspnea (R2 = 0.38, p = 0.04) and the decline of RVLS_FW (R2 = 0.53, p = 0.02). Notably, compared with the other echocardiographic parameters only RVLS_FW at the early stage (T2) significantly correlated with the development of dyspnea (odds ratio: 1.84, 95% confidence interval: 1.22-2.78, p = 0.04). Conclusions: RVLS_FW sensitively predicts dyspnea development in breast cancer patients receiving epirubicin therapy. However, larger scale studies are required to validate its role in long-term patient survival.