Because of the paucity of information regarding long-term effects of exercise, and because of the increased emphasis in the scientific community on evaluating the long-term efficacy of exercise interventions for osteoporosis prevention, this comprehensive 1-year clinical trial and 2-year followup will make an important contribution to determining the future role of combining progressive resistance and weight bearing aerobic exercise in the prevention of osteoporosis in two populations (HRT vs. no HRT) of postmenopausal women.