In the United States and many Western nations, increasing prevalence of osteoporotic fracture is contributing to the health care system burden, and costs and needs for medical services are predicted to increase significantly by the beginning of the 21st century. Recent reports investigating developmental changes in skeletal mass of adolescent girls and young women under different experimental or ecological conditions support the contention that modification of environmental factors, especially dietary calcium and physical activity, can favorably modulate bone mass and bone density compared to controls. The peripubertal period, starting as early as 10 years of age, seems to be most responsive to modification of environmental/lifestyle factors, whereas potential gains of bone mass during late adolescence and early adulthood, although smaller, may be more readily achieved through improved dietary calcium intakes and regular exercise programs. Scientific evidence in support of these beneficial effects on bone is presented as part of the rationale for prevention of osteoporotic fractures. Also discussed is the need for a national policy to prevent osteoporosis through primary prevention strategies focused at young girls prior to puberty. The goal would be for every prepubertal girl, over the next decade, to achieve peak bone mass (and density) of the spine, hips and other bones by age 20, i.e., early adulthood. A second target group for national policy should be women in their 20s. Attainment of the goal to gain 3-5% additional bone mass during this decade would prolong the time before the fracture threshold range (low bone mass) is reached in the postmenopausal decades.