Introduction Early detection of left ventricular systolic dysfunction (LVSD) is important for therapeutic strategies for Duchenne muscular dystrophy (DMD) patients. We analyzed myocardial strain using echocardiography for early detection of LVSD and determined the predictors of early LVSD. Methods This investigation was a cross-sectional study of 40 DMD patients with normal left ventricular ejection fraction. Global longitudinal strain (GLS) was used to analyze subtle disturbances in longitudinal contraction of the myocardium. Patients were determined to have early LVSD (GLS > -18) or normal left ventricular systolic function (GLS ≤ -18). Results Patients who had early LVSD were older and had a higher frequency of corticosteroid therapy and of mutations in exons 45, 46, 47, 48, 49, 50, and 52. Discussion Myocardial strain measurements are useful for the early diagnosis of LVSD in DMD patients. Older age, use of corticosteroids, and mutations within the "hot-spot" region of the DMD gene are associated with early LVSD. Muscle Nerve, 2018.