Study of Breast Cancer Prevention by Letrozole in High Risk Women

Inclusion Criteria: * Post-menopausal women at high risk for development of breast cancer * stable dose of hormone replacement therapy * have cytomorphologic evidence of hyperplasia +/- atypia and Ki-67 expression \>1.5% in benign breast epithelial cells acquired by RPFNA * Serum level of 25-hydroxyvitamin D of at least 30 ng/ml prior to study entry * Willing to have a repeat random periareolar fine needle aspiration (RPFNA) and mammogram at 6 months and 12 months (if participating in the open label portion of the study) following initiation of study drug Exclusion Criteria: * Prior history of osteoporosis or osteoporotic fracture. * Prior history of invasive breast cancer or other invasive cancer within five years from date of study entry. * Current and chronic use of cyclooxygenase-2 (COX-2) specific inhibitors or NSAIDs * Receiving treatment for rheumatoid arthritis or fibromyalgia * Current history of poorly controlled migraines or perimenopausal symptoms * Currently receiving other investigational agents. * Receipt of more than 6 months of an aromatase inhibitor (anastrozole, exemestane, letrozole, etc.) at any time in the past.