BACKGROUND: Increases in juvenile overweight and obesity have emphasized the importance of accurate and accessible body composition (BC) assessment, namely for the monitoring, prevention and treatment. Bioimpedance analysis seems to be one of the simple, safe, and inexpensive methods for assessment of the body composition in paediatric subjects METHODS AND RESULTS: Three-thousand-twelve children aged 6-14 years (1611 - 53.5 % boys, and 1401 - 46.5 % girls) were evaluated BC using mono frequency and multi-frequency bioimpedance analyser with adapted prediction equation for Czech children. In both genders %BF decreased significantly with age (r = -0.698 in boys and r = -0.681 in girls, both p<0.0005). Linear regression analysis showed a significant positive relationship between %BF measured with BMI %BF(%)= 0.921*BMI(kg.m(-2)) -4.292, r = 0.858, r(2) = 0.736, SEE = 1.22 % in boys, and in girls %BF(%) = 1.284*BMI(kg.m(-2)) - 6.906, r = 0.903, r(2) = 0.815, SEE = 1.05 %, p<0.0001 in both cases. CONCLUSIONS: BIA method is a useful method for detecting BC in children and can be accepted as a precise tool for measuring of BC in epidemiological studies in children. The decisive role for the use of this method has the population oriented prediction equation and the subject's hydration state.