Background: Vitamin D is safe and has breast cancer prevention properties. CALGB 70806 was a randomized phase II trial evaluating the effect of vit D on several breast cancer biomarkers (including MD and serum IGF1). Methods: Premenopausal women were assigned to receive either 2000IU of Vit D or placebo for 12 months, stratified by baseline (BL) vit D level (sufficient vs insufficient). Eligible women were premenopausal, age < 55, with at least 25% dense breast tissue. Biomarker specimens were collected at baseline and 12 months. MD was determined using the Breast Imaging Reporting and Data System (BIRADS), semi-automated and automated methods. Serum IGF1 was determined by ELISA. Biomarkers were compared between arms using Wilcoxon and t-tests. Results: 300 women were recruited from 41 institutions across the US between 1/11 -12/13. The mean age was 42.6 years with 14% Hispanic, 12% African American, 74% European. 62% of participants were vitamin D deficient at enrollment and 49% of women had MD between 25-50% with only 12% over 50% dense. 216 (72%) of participants completed treatment, 8 withdrew due to side effects, and 76 for other reasons (28% withdrawal rate). A significant increase in Vit D was seen with treatment with 99% and 72% of experimental and control subjects having sufficient levels at 12 months(P < 0.0001)). MD decreased 2.2% over 1 year for the entire cohort, with no significant difference between arms (Table 1). Similarly, no significant change in IGF1 levels was seen with Vit D. Conclusions: Vit D supplementation resulted in a significant increase in serum Vit D (from a mean of 35.5 to 49.7 ng/mL, p = < 0.0001). However, no significant change in MD was observed with treatment; potentially due to small change in MD seen at 1 year, the low percentage of high MD or that Vit D works by another mechanism. Further study with longer Vit D exposure is warranted. Support: UG1CA189823, U24CA196171. Clinical trial information: NCT01224678