Higher vitamin D levels at diagnosis are associated with lower risk of mortality in colorectal cancer patients. Inflammation is one of the possible underlying mechanisms. Therefore, the overarching objective of this thesis was to investigate associations between vitamin D concentrations, inflammation and their interplay in relation to colorectal cancer (CRC) outcomes. Overall, in this thesis we observed an association between vitamin D and CRC outcomes, where higher vitamin D concentrations over time were associated with a lower risk of all-cause mortality, especially when combined with a magnesium-rich diet. Higher vitamin D concentrations were associated with lower levels of inflammatory markers, while higher levels of these markers were associated with worse CRC outcomes. The role of vitamin D in cancer recurrence is still unclear. The anti-inflammatory effects of vitamin D as one of the potential underlying mechanisms for its association with CRC outcomes should be further explored using a molecular pathological epidemiological research approach.