Colorectal cancer (CRC) patients with pre-existing diabetes have significantly lower rates of overall survival compared with patients without diabetes. Against this backdrop, the American Diabetes Association and American Cancer Society in 2010 reviewed the scientific literature concerning diabetes and cancer. One of the key issues identified for further investigation was the need for a better understanding of whether diabetes influences cancer prognosis above and beyond the prognosis conferred by each disease state independently. Whether the worsened survival of CRC patients with diabetes could be explained by less favourable patient-, tumour- and treatment-related characteristics has also been evaluated in numerous recent studies. However, as most studies did not account for all the various potential confounders, such as cancer stage, comorbidities and body mass index (BMI) in their analyses, the current evidence for the association between diabetes and survival in CRC patients remains inconclusive. Nevertheless, based on multiple examples in the literature, the present review demonstrates that diabetes affects the presentation of CRC as well as its treatment and outcome, which may then result in lower overall rates of survival in patients with, compared to those without, diabetes.