Based on indications of a possible prognostic role of prostaglandin endoperoxide synthase-2 expression (PTGS2) in colorectal cancer, a systematic review was conducted to assess the prognostic significance among colorectal cancer patients. Despite indications of an association between PTGS2 expression and tumour recurrence, there was insufficient evidence to indicate an independent association between PTGS2 expression and colorectal cancer prognosis. Previous studies had indicated a potentially large prognostic benefit associated with use of low-dose aspirin following a diagnosis for colorectal cancer, but could be subject to reverse causality. Therefore, the association between low-dose aspirin use and survival among colorectal cancer patients was assessed using a design to reduce reverse causality. The results did not support an association between low-dose aspirin use and survival in colorectal cancer patients and suggested reverse causality was likely in previous studies. A systematic review with meta-analyses was conducted to assess the prognostic significance of vitamin D-related factors such as vitamin D status among colorectal cancer patients. The pooled results from all primary studies identified suggested that vitamin D status appears to be associated with coloreclal cancer survival. A prospective cohort analysis was used to assess the association between intakes of fibre and fruits and vegetables with risk of colorectal adenomas and cancer in screened individuals. The analyses provided evidence that fibre, but not fruit and vegetable, intakes were associated with a reduced risk of incident colorectal adenoma. Overall the thesis findings support a potential prognostic benefit associated with vitamin D concentrations, but not tumour PTGS2 expression or aspirin use among colorectal cancer patients. The findings also indicate that dietary fibre may act early in the development of colorectal cancer by reducing risk of incident colorectal adenomas