NCI is examining the association between prospectively collected circulating 25-hydroxyvitamin D [25(OH)D] levels and risk of breast and colorectal cancer in a consortium of 16 cohort studies from North America, Europe and Asia. Together, these cohorts will have 25(OH)D levels measurements in over 5,000 colorectal cancer cases and over 6,800 breast cancer cases. These analyses should contribute substantially to characterizing the dose-response relationships of these associations and will inform recommendations regarding sun exposure, vitamin D intake requirements, and design issues for any future randomized trials.