The structure, shape, size, growth and mineralization of bones are results of constant bone creation and its' resorption, described as bone turnover. The two first decades of life, when the peak bone mass is gained, seems to be the most important for further skeletal changes. Thus closer examination of such factors as gender, weight, height, bone age, pubertal status, lean body and fat mass, as well as environmental factors at particular age should be conducted. Since there are no clear criteria for small bone mass in the population of children and adolescents, an even more complicated situation for establishing bone status is with the cohort of children with chronic diseases. Childhood cancer and its' treatment are additional negative influences for bone gain and therefore it could contribute to osteopenia and osteoporosis in the adult.