Osteoporotic fractures associated with low bone mineral density (BMD) are a significant public health problem for women, resulting in over 1 million fractures per year. The increasing incidence of fractures will make an already major health problem an even greater one in the future. Estimates of medical costs for the treatment of osteoporotic fractures are expected to increase from $14.8 billion in 1995 to $60 billion by 2020. Although the Women's Health Initiative is investigating dietary and hormonal factors in relation to bone density, it is not investigating the effects of exercise on reducing bone mineral loss in postmenopausal women.