507 Background: Prospective epidemiologic data suggest that higher levels of 25-hydroxyvitamin D [25(OH)D] are associated with improved survival in patients with colorectal cancer (CRC), however, the relationship between 25(OH)D and outcome in metastatic CRC, specifically, is unknown. Methods: We prospectively assessed the association between plasma 25(OH)D and overall survival (OS) in previously untreated metastatic CRC patients enrolled in CALGB 80405, a randomized phase III trial of chemotherapy + bevacizumab, cetuximab, or both, prior to the KRAS WT amendment. Progression-free survival (PFS) was a secondary endpoint. Plasma 25(OH)D levels were measured at baseline by radioimmunoassay, and dietary and lifestyle behaviors collected from self-administered questionnaires. Cox proportional hazards models were used to calculate hazard ratios adjusted for other prognostic factors. In sensitivity analyses, patients who died within 3 or 6 months of blood draw were excluded to address the possibility of reverse...