2. To determine bone density and bone turnover metabolites in post-menopausal women. The bone densitometry will be done at baseline and following 24 months of letrozole or exemestane therapy. The bone turnover metabolites will be done at baseline, three, six and 24 months following letrozole or exemestane therapy. These data will allow us to test the hypothesis that variants in candidate genes involved in estrogen metabolism or signaling alter the ability of exemestane or letrozole to bring about changes in bone.