Introduction and objective: Myocarditis, due to its diverse clinical presentation, is a big challenge for clinicians. Aim of the paper was to assess the left ventricular myocardial performance index by echocardiography and the relationship between left ventricular myocardial performance index and the oedema in T2-weighted cardiac magnetic resonance imaging in children with myocarditis. Materials and methods: The retrospective study included 32 patients with diagnosed myocarditis, in whom echocardiographic parameters, including left ventricular myocardial performance index, were compared with cardiac magnetic resonance imaging results. The control group consisted of 30 teenagers. Results: Statistically significant differences were obtained between the mean values of the left ventricle myocardial performance index in the study group compared to the values in the control group for the basal segment of the interventricular septum, respectively 0.52 ± 0.07 vs. 0.4 ± 0.08 (p < 0.001), the basal segment of the lateral wall 0.48 ± 0.02 vs. 0.41 ± 0.08 (p = 0.019) and the middle segment of the interventricular septum 0.5 ± 0.01 vs. 0.39 ± 0.09 (p = 0.001). In 12 patients the average value of the left ventricle myocardial performance index was <0.5, in 8 of them (66.6%), no residual changes were observed on cardiac magnetic resonance after 9 ± 2 months. All patients in whom average left ventricle myocardial performance index was >0.5, residual changes persisted in cardiac magnetic resonance. Conclusions: In children with myocarditis, the values of the left ventricle myocardial performance index in echocardiography are increased. Higher values of left ventricle myocardial performance index indicate a longer duration of the inflammatory process, which is reflected in cardiac magnetic resonance imaging.