Background: Breast cancer is a hormonally dependent cancer and a woman's serum estrogen and androgen concentrations are positively associated with risk. Breast density is one of the strongest risk factors for breast cancer, and serum hormones could potentially influence breast cancer risk via effects on breast density. However, postmenopausal endogenous estrogen and androgen concentrations generally are not associated with breast density, and although positive associations have been reported for premenopausal women, results are inconclusive. Most of the participants in studies of premenopausal hormones and breast density were 40 years or older and well past the time of breast development, which occurs primarily during adolescence when estrogens and progesterone regulate elongation and branching of breast ducts and could have their greatest effects on breast density. Thus, we propose to take advantage of a unique opportunity to prospectively evaluate associations of adolescent serum sex hormone levels with breast density using resources previously collected as part of the Dietary Intervention Study in Children (DISC) and the DISC06 Follow-Up Study. Objectives/Hypothesis: We hypothesize that higher sex hormone levels during adolescence are associated with greater breast density in young adulthood. Specific Aims: Primary aims are to determine if adolescent serum estradiol, testosterone and progesterone concentrations are associated with percent breast density and volume of dense breast tissue in young adult women. Study Design: Previously collected resources in DISC and the DISC06 Follow-Up Study will be used. DISC was a randomized controlled trial in which healthy, prepubertal 8-10 year olds with elevated LDL-cholesterol were randomly assigned to a behavioral intervention to lower fat and increase fiber intake or to a usual care control group. The intervention continued for an average of 7 years and was terminated when participants' mean age was 16.7 years. Blood was collected for analysis of serum biomarkers including sex hormones at baseline, Year-1, Year-3, Year-5 and last visits. Follow-up visits for female participants to evaluate longer-term effects of the DISC intervention on biomarkers associated with breast cancer including breast density took place 9 years after termination of DISC when participants were 25-29 years old. We will use multilevel linear models to evaluate associations of adolescent serum sex hormone levels with breast density in 177 DISC participants who had hormones measured on one of more occasions during adolescence in DISC and who had breast density measured at the DISC06 follow-up visit. Significance: We are aware of no prior studies that have prospectively evaluated associations of adolescent serum hormone levels with breast density in young adulthood as we propose. Findings will add importantly to our understanding of the influence of adolescent sex hormone levels on breast density and breast cancer risk.