Abstract Background In a pilot study, a structured program of reduced energy diet, physical activity, and weekly group behavioral intervention with other high risk women was successful at producing a median 11% weight loss with at least a 5% weight loss in 88% of subjects. We evaluated the association of weight loss with changes in serum and breast tissue risk and mechanisms of action biomarkers. Methods High risk postmenopausal women with BMI >25 kg/m2 had breast tissue harvested by random periareolar fine needle aspiration (RPFNA) before and after a 6-month energy balance intervention. Specimens were evaluated for biomarkers including cytomorphology, proliferation (immunocytochemical Ki-67), gene expression by RT-qPCR, and expression of cytokines and adpokines by Luminex assay. Fasting serum was assayed for insulin, glucose, adiponectin, leptin, high sensitivity CRP, IL-6, prolactin, SHBG, estradiol and testosterone using ELISA or Luminex. Results For 24 biomarker evaluable subjects, 21 had >5% weight loss (median = 11%). Cytologic atypia was present in 10/24 at baseline and 4/24 at 6 months (p=0.034). For 20 subjects with sufficient cells for assessment of Ki-67 at both times, median baseline Ki-67 was 0.7% and off study 0.3%, with a median change of -0.2% (p=0.19). Statistically significant changes (≥0.003; Wilcoxon) were observed for serum levels of adiponectin, adiponectin:leptin ratio, and SHBG (increases); and leptin, bioavailable estradiol and hsCRP (decreases). Reduction was also observed for insulin (p=0.018) and bioavailable testosterone (p=0.033). These results were duplicated (p≥0.014) by Luminex for adiponectin, leptin, adiponectin: leptin ratio, and insulin; plus hepatocyte growth factor (HGF, decrease). Also, an increase in the adiponectin:leptin ratio was observed for the RPFNA specimens (p=0.012). Gene expression (RT-qPCR) of pS2 was significantly modified (decrease, p=0.035). Further, the weight loss (expressed as relative change) was highly statistically correlated with change (relative) in serum leptin, adiponectin:leptin ratio, SHGB, and free estradiol; as well as with relative change in adiponectin:leptin ratio in RPFNA specimens. Conclusion Weight loss in high risk postmenopausal women is accompanied by significant modulation of numerous serum and breast tissue-based biomarkers. For several risk and response biomarkers there is a significant correlation between change in the biomarker and the weight loss achieved. This suggests the possibility of identifying mechanisms of action and signaling pathways for dietary/energy balance interventions that may reduce risk for development of breast cancer. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD09-04.