Patients undergo blood sample collection at baseline and at 1 year after initiation of aromatase inhibitor therapy (anastrozole or exemestane). Samples are analyzed for estrogen and testosterone levels and additional hormone levels and growth factors that have been previously linked with breast density and that could be altered by aromatase inhibitor use (i.e., sex hormone-binding globulin [SHBG], DHEA, DHEA sulfate, progesterone, prolactin, insulin-like growth factor-1 [IGF-1], and insulin-like growth factor binding protein 3 [IGF BP3]). Samples are also analyzed for anastrozole and exemestane levels by HPLC. Pharmacogenetic studies are also performed. Haplotype-tagged single nucleotide polymorphisms in genes in the aromatase pathway are examined.