(18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging is highly accurate in restaging colorectal cancer, esophageal cancer, and gastrointestinal stromal tumors. Overall, it compares favorably with anatomical imaging in the evaluation of tumor recurrence because metabolic abnormalities usually precede a structural change. Initial staging of these malignancies with PET is best used in patients with locally advanced disease who may benefit from curative resection if distant metastases are not found. It also appears to have great potential in predicting histopathologic response to neoadjuvant therapy and in monitoring the success of radiofrequency ablation and (90)Y microspheres radioembolization soon after intervention. FDG-PET can be used in other gastrointestinal malignancies as a prognostic tool and to detect distant disease but its role has not yet been well defined.