Mammographic density (MD) is one of the strongest known risk factors for breast cancer. Our primary goal is to identify novel genes for MD by conducting a genome-wide association study (GWAS) in 30,000 Caucasian women participating in KPNC’s Research Program on Genes, Environment and Health (RPGEH). We will obtain routine screening mammograms and assess breast density. RPGEH and KPNC databases will provide genome-wide marker data for over 650K SNPs as well as extensive epidemiologic and medical data, including BMI, lifestyle and reproductive factors, and use of prescription medications.