The risk of osteoporosis is directly related to factors that influence bone remodeling premenopausally and postmenopausally. Some of these factors are nonmodifiable--for example, race and the genetic control of osteogenesis. Other factors, such as lifestyle, exercise and nutrition, can be influenced to enhance bone mineral accrual and thereby lessen the risk of or even prevent osteoporosis. Central to the health of the bone-remodeling cycle is the role of the sex steroids estrogen and androgen, and their bioavailability to estrogen and androgen receptors in the osteoclasts, osteoblasts and osteocytes. Apart from their direct action on bone cells, sex steroids modulate the bone-preserving function of modifiable factors. The efficacy of sex steroids is proportional to their bioavailability in the "free" form. This is equally true of endogenously synthesized hormones and of hormone replacement therapy.