It is believed that peak bone mass is a critical element in the determination of risk for osteoporotic fractures. At the time of peak bone there is a 1 SD black-white difference in bone mass which must contribute greatly to the 50% reduction in osteoporotic fractures among elderly black men and women in the U.S. This difference in peak bone mass must reflect processes which differ in black and white children during skeletal growth, and it is the purpose of Part A of this proposed research to investigate: l) the black-white differences in rates of gain in bone mass in children aged 5-14; 2) the relationship between markers of skeletal turnover and rates of gain in bone mass in black and white children, and whether such differences account for differences in skeletal growth; and 3) factors which might influence bone mass either through effects on skeletal modeling and remodeling during growth, or through other mechanisms, including sex steroids, growth hormone and other growth factors. In Part B we propose to also study the role of skeletal remodeling in bone mass and bone loss differences among black and white adults. We will examine both differences in bone mass and rates of change in bone mass in older black and white men and women, and investigate whether changes in rates of skeletal remodeling are associated with rates of change in skeletal mass. We will also test whether there are differences in sex steroids which are associated with differences in bone mass, bone loss, markers of remodeling, or other potential influences on the skeletal mass (e.g. muscle mass).