This is a multi-institution double-blind placebo-controlled trial whose main objective is to determine if 6 months of letrozole (2.5 mg daily) can reduce proliferation as assessed by Ki-67 in high risk postmenopausal women on systemic hormone replacement therapy who have random periareolar fine needle aspiration (RPFNA) evidence of hyperplasia with atypia or borderline atypia, and a minimum Ki-67 of >1.5%.