In 72 children having a body surface area from 0.8 to 1.6 m2 who systematically trained in swimming at last for 1 year, or an average of 29.2 months, echocardiographic recordings were done to find out which changes result from systematic training. The results obtained were compared with values obtained in 72 normal children, having the same body area, but who did not actively engage in sports. The measurements and analyses showed that values of the left ventricular interior diameter (LVID), left atrium (LA), and heart depth (DS) were statistically significantly increased. The values of septum thickness (SE) and left ventricular posterior wall (LVPW) were a bit larger, but statistically not significant. Contractibility of the myocardium was also examined by measurement of the ejection fraction (EF), percentage of the shortening of the left ventricle (%FS), and average velocity of the circumferential shortening of the muscular fibers (VCF). The values obtained were within limits of normal values. Results of the analysis of the influence of sports training on echocardiographic dimensions indicated four positive correlations: left ventricular interior diameter in systole (LVIDs) and diastole (LVIDd), diameter of the left atrium (LA), and heart depth (DS). In recording echocardiograms in children, it is necessary to establish whether the child actively engages in sports beforehand since long-term training leads to changes of some heart structures, which may, if it is not known that the child is active in sports, lead to wrong conclusions that the finding is pathological.