Over last decades in the developed countries, the gain in overweight and obesity both among the adults and children’s population is noted. Obesity in children and adolescents is a predictor of cardiovascular accidents at an older age. Understanding characteristics of the body composition in children with the obesity will increase the effectiveness of therapeutic interventions in the correction of obesity and prevention of the formation of the cardiovascular risk. 181 children aged 10-17 years participated in the study executed in 2011-2015. Anthropometry and bioelectrical impedance analysis were performed in all children. According to BMI, all children were divided into 3 groups: the overweight, obesity and control group. An analysis of the ratio of actual values of body components to the height squared showed a significant gain in all body components to be observed in the group of obese children compared to the control, but the increase in active cell mass (ACM) was significantly lower in comparison with changes in fat mass. Thus, there is a decrease in the number of metabolically active cells in obesity relative to body mass. Analysis of fluid components in examined groups of children revealed a decrease in extracellular fluid sector relative to ACM observed in obesity, which may indicate to the decreased volume of circulating blood plasma and the increase in the blood viscosity. At the same time, a significant extracellular and intracellular fluid deficiency was noted in the adipose tissue in the group with the obesity. Thus, we have not obtained data on the water retention in the development of the obesity in children and adolescents, which may be important for understanding the genesis of hypertension, often associated with the obesity. The fluid deficit occurring in obesity should be taken into account in the appointment of drinking water regime and diuretics.