Maturation has profound effects on bone mineral accrual. Prior to the appearance of the physical signs of puberty the only available method of assessing relative maturity or "biological age" is to measure skeletal or "bone age" by standardized x-rays. Skeletal age continues to be of use, even after the appearance of the physical signs of puberty, because the rate of progression of bone maturation with puberty varies between individuals. It is particularly variable in children with chronic disease whose general health status and rates of growth and pubertal progress may "catch-up" or "catch-down" in response to changes in treatment or in disease activity. In children with chronic disease, bone age may be a better indicator of maturational status than chronological age, size, or pubertal stage. Bone age therefore an important co-variate in this protocol.