Myocardial dysfunction in children diagnosed with mitochondrial disease is an ominous sign and has been associated with substantial increased mortality rates. Early detection of cardiac involvement would therefore be desirable. Two dimensional strain echocardiography (2DSTE) has proven to be more sensitive than conventional echocardiography for the detection of early myocardial dysfunction in various (cardiac) conditions.To determine left ventricular systolic function in children with mitochondrial disorders by means of physical examination, electrocardiography (ECG), conventional echocardiography and 2DSTE.A total of 27 children with established mitochondrial disease and 54 age-matched control subjects underwent cardiac evaluation. Longitudinal, circumferential and radial peak systolic strain (S) values were determined as well as peak systolic strain rate (Sr) and the time to peak global systolic strain (T2P). One Way analysis of Variance was performed to assess the influence of the presence of mitochondrial disease on conventional echocardiographic and 2DSTE outcomes.Conventional echocardiographic findings did not indicate systolic left ventricular dysfunction. Global peak S, Sr and T2P measurements in all three directions were significantly lower in children with mitochondrial disease (P<0.001) when compared to controls.2DSTE detects alterations in myocardial systolic function in children diagnosed with mitochondrial disease, whose conventional echocardiographic findings did not indicate ventricular systolic dysfunction.