The present paper is the first attempt at such a comprehensive analysis of the echocardiographic evaluation of the structure and function of the left ventricle (LV), as well as electrocardiographic evaluation of the time of the myocardium functional potential performed in 60 persons (30 ♀, 30 ♂) with type 1 diabetes mellitus (DM1) in age of puberty (14-17 years) and in 30 persons (15 ♀, 15 ♂) of the control group. The analysis were performed separately for both sexes and the results were analyzed in reference to degree of the metabolic control in short (3 months), and long (several years) period of time. In DM1 patients, independently from sex, there was statistically significant extension of the isovolumetric relaxation time (0,06 vs 0,056 s, p < 0,05), not statistically significant increase of the A wave value (60,3 vs 58 cm/s in ♀and 56,12 vs 53,95 cm/s in ♂), deceleration time (0,17 vs 0,15 s and 0,16 vs 0,15 s) and decrease of the E wave value (99,67 vs 101,8 cm/s and 97,35 vs 104,9 cm/s) and E/A (1,7 vs 1,78 and 1,79 vs 2,0). There was no systolic function disorders. The diastolic dysfunction of left ventricle was dependent from the degree of insulin resistance, and DM1 duration in girls, and from metabolic control status in long period of time on diastolic function of heart in boys. There were tachycardia and QTc elongation in ECG observed in DM1 patients (0,4 vs 0,36 s, ; p<0,01 in ♀, 0,39 vs 0,35 s in ♂, p<0,01 ), independent from sex and DM1 duration, but dependent from metabolic control status in last 3 months. The observed changes are early demonstration of the diabetic cardiomyopathy, which is the main cause of the morbidity and mortality in later age. That may be important for the prognosis of the next component of this disorder at individual diabetic patients as well as for the improvement of the early prevention methods.