b＞Backgrounds: /b＞Mitochondrial disease (MD) and Duchenne muscular dystrophy (DMD) are often associated with cardiomyopathy and high mortality. Studies have not verified the myocardial variability within a certain characteristic as skeletal muscle wasting. We evaluated left ventricular (LV) mass by echocardiography to identify muscle mass distribution and its relation to cardiac function.br＞ b＞Methods and Results: /b＞We retrospectively evaluated the echocardiographic data of 90 MD and 42 DMD patients between 2.5 and 18-years-old. Using two-dimensional echocardiography, including time-motion (M) mode and Doppler measurements, we estimated ejection fraction, LV mass, relative wall thickness, Mitral ratio of early to late filling velocities (E/A), early filling velocities to early diastolic annular velocity (E/Ea), stroke volume, and cardiac output. A “z score” was generated using the lambda-mu-sigma method to standardize LV mass for body size. LV mass-for-height z scores were significantly below normal in children with MD (-1.02; 95% confidence interval, -1.34 to -0.71) or DMD (-0.82; 95% confidence interval, -1.32 to -0.32), as were LV mass-for-lean body mass z scores. Comparison of relative wall thickness showed significant difference between MD and DMD (0.38 versus 0.35; P=0.025). LV mass-for-height z score had positive correlations with stroke volume index (R=0.434; P0.001) and cardiac index (R=0.316; P0.001), as did LV mass-for-lean body mass z score.br＞ b＞Conclusions: /b＞We calculated LV mass z scores and showed that LV myocardial atrophy is present in MD and DMD patients, but there might be different pathophysiologic processes. Decreased LV mass within the same body size might indicate deterioration of cardiac functional reserves in patients with MD and DMD.