Аннотация:The epidemic of overweight and obesity that has affected the world's population hasbecome a great concern. Excess fat mass is associated to metabolic syndrome and cardio diseases.The body mass index is widely used to measure adiposity in adults. The WHO has proposed to usethe BMI to assess the fat mass in children too. But the proportion of fat mass and fat free mass inchildren is an important determinant of fitness and health. New indices have been proposed for theanalysis of body composition: fat mass index and fat free mass index.The aim of this study was to describe the age and sex specific changes of the FMI andFFMI in Russian schoolchildren and to analyze their relationship with BMI.In a cross-sectional study of physical development of Russian children and adolescentswhere collected data from 3873 participants aged 7 to 17 years old of both sexes in schools fromMoscow, Arkhangelsk and Arkhangelsk region. Body composition was measured by a multyfrequency, tetra-polar bioelectric impedance analyzer Medass.BMI increases throughout 7 to 17 years. It is thought BMI age changes should reflectchanges in FM. But we have seen the FM changes do not follow the expected curve of BMIchanges. In boys FFMI has a higher growth rate than FMI. In girls FMI has a higher growth ratethan FFMI.FMI more accurately describes age-related changes in fat mass and sex differences.