Colorectal cancer is an important cause of cancer morbidity and mortality. Despite improvements in therapeutic strategies, cancer recurs in a significant number of these patients. Accurate preoperative staging at initial presentation and at recurrence are needed to select those patients most likely to benefit from surgery. Conventional imaging studies are insufficiently accurate for these purposes. The addition of the functional information derived from positron emission tomography (PET) with F-18 fluorodeoxyglucose has been shown to be an important adjunct to conventional imaging methods in evaluation of colorectal cancer. PET has the ability to detect disease in structures that appear normal on conventional imaging studies. It has been shown to be extremely useful for defining the full extent of recurrent or distant metastatic disease, particularly in patients with a high clinical suspicion of disease and normal conventional imaging studies. The use of PET to identify patients who are not suitable candidates for curative surgery has been shown to be cost-effective. In addition, PET has been shown to be useful in assessing response to therapy and differentiating post-therapeutic changes from recurrent or residual disease.