Adolescence is a period of rapid skeletal growth during which nearly half of the adult skeletal mass is accrued. This life stage is a window of opportunity for influencing peak bone mass and reducing the risk of osteoporosis later in life. Endocrine factors that may influence peak bone mass include insulin-like growth factor-1, which regulates skeletal growth, and gonadotropic hormones, which stimulate epiphyseal maturation. Estrogen deficiency and amenorrhea can reduce skeletal mass. Weight-bearing exercise can increase bone mass. Appropriate mineralization of the skeleton requires adequate dietary intakes of minerals involved in the formation of hydroxyapatite; the most likely to be deficient is calcium.