The purpose of this study was to determine the differences in the left ventricular dimensions, mass index and function in school-aged swimmers by echocardiography. The study group consisted of 82 swimmers who participated in a systematic swimming training for at least six months, and the control group consisted of 41 sedentary children of similar age, sex and weight. In the study group, left ventricular dimensions and wall thicknesses, aortic root and left atrium diameters, and left ventricle mass index were significantly greater than the normal children (p < 0.05). However, there was no difference in the left ventricular systolic function (ejection fraction, shortening fraction) or in the left ventricular filling characteristics (p > 0.05). In conclusion, in childhood swimmers there was a significant increase in left ventricular dimensions, wall thicknesses and mass index, but no differences in the systolic function and filling characteristics of the left ventricle. Thus, information on endurance training participation is necessary to interpret quantitative echocardiographic data.