Background: Animal and ecologic studies strongly support a role of diet in breast cancer etiology, but case-control and cohort study results are equivocal. Rapid growth and maturation of the breasts occurs during adolescence, and a woman's diet as an adolescent may affect her risk of developing breast cancer more than her diet as an adult. The Dietary Intervention Study in Children (DISC) was a randomized controlled clinical trial that evaluated the effect of a reduced fat diet intervention during puberty on serum sex hormones in 301 girls who were healthy 8-10 year olds at randomization. After 5 and 7 years of participation, respectively, the intervention group had significantly 30% lower follicular phase serum estradiol and 53% lower luteal phase progesterone levels compared to the usual care group. Estradiol and progesterone are breast mitogens that regulate breast development during puberty and these differences could potentially influence participants' risk of developing breast cancer as adults. We propose to take advantage of this unique opportunity to evaluate the effect of a diet intervention during adolescence on biomarkers associated with breast cancer risk to improve our understanding of breast development and the origins of breast cancer. Specific Aims: Our primary aim is to determine the long-term effect of the DISC intervention on serum progesterone levels in early adulthood. Secondary aims are to determine the effects of the DISC intervention on serum estradiol levels, bone mineral density, and breast density in early adulthood. Design: A follow-up visit will be conducted with the 301 girls who participated in DISC and who are now in their twenties. Data will be collected on demographics, medical history, menstrual and reproductive history, physical activity, and diet. Heights and weights, serum progesterone and estradiol levels, bone mineral density, and breast density will be measured. Significance: The proposed study will determine, as part of a controlled clinical trial, the effects of diet during adolescence on breast cancer risk via effects on serum hormones, breast density, and bone mineral density. These biomarkers are three of the strongest risk factors for breast cancer and could readily be affected by adolescent diet. Because diet is a modifiable risk factor, results of this proposed study could be used to develop strategies for prevention.