Peak bone mass, which can be defined as the amount of bony tissue present at the end of the skeletal maturation, is an important determinant of osteoporotic fracture risk in adulthood. The techniques of single or dual energy absorptiometry measure the so-called "areal" or "surface" bone mineral density (BMD), a variable which has been shown to be directly related to bone strength. During puberty the gender difference in bone mass becomes expressed. This difference appears to be essentially due to a more prolonged bone maturation period in males than in females, with a larger increase in bone size and cortical thickness, as there is no significant sex difference in the volumetric trabecular density at the end of pubertal maturation. At the beginning of the 3rd decade, there is a large variability in the normal values of areal BMD in axial and appendicular skeleton. This large variance, which is observed at sites particularly susceptible to osteoporotic fractures in adulthood, such as lumbar spine and femoral neck, is barely reduced after correction for statural height, and does not appear to substantially increase during adult life. It is generally accepted that peak bone mass at any skeletal site is attained in both sexes during the mid-thirties. However, recent studies indicate that in healthy caucasian females, bone mass accumulation can virtually be completed before the end of the second decade, for both lumbar spine and femoral neck. Several variables are supposed to influence bone mass accumulation during growth: heredity, sex, diet components, endocrine factors, mechanical forces, and exposure to risk factors.(ABSTRACT TRUNCATED AT 250 WORDS)