We started the Bone, Estrogen, Strength Training (BEST) study in the fall of 1995 at the University of Arizona, Tucson, Arizona. The BEST study is the largest of its kind. It looks at the effects of hormone replacement therapy and strength training exercise on bone mineral density. (Bone mineral density affects bone strength and the risk of osteoporosis.) Six groups of about 300 women each participated in this osteoporosis prevention study. In 1998, the BEST study received additional funding to examine for another 2 years the long-term effects of strength training on bone mineral density. By 2001 we will have finished analyzing the results for all study groups on the 1-year effects of exercise on bone, as well as additional analyses on the effects of 2, 3, and 4 years of strength training and weight-bearing exercise on bone.