PET is a diagnostic method that creates high resolution, 3 dimensional tomographic images of the distribution of positron emitting radionuclides in the human body. Recent technological developments allow the use of whole-body PET devices in clinical oncology. 18FDG is a glucose analog transported and competitively used with glucose reflecting the increased glucose metabolism into malignant cells. Differential diagnosis between chronic pancreatitis and pancreatic cancer is already a well-documented indication. For initial staging of gastro-esophageal and colorectal tumours, results are preliminary but the clinical impact seems to be rather limited. At present, the major indication of FDG-PET is the detection and staging of colorectal cancer recurrences. FDG-PET allows the differentiation between scared tissue and tumour when structural imaging is often confusing. In the same time, the whole-body imaging capability provides unique information that can modify loco-regional and liver staging. Overall, FDG-PET affects the clinical management of 30 to 40% of these patients. Quantitative assessment of therapeutic response to chemotherapy regimen appears to be one of the most promising applications of FDG-PET. Since the most effective therapy of colorectal cancer are often surgical, the role of chemotherapy in colorectal cancer remains limited to adjuvant therapy and in advanced disease. However, FDG-PET could be of great value in assessing the response of oesophageal carcinomas to chemo-radio therapy, before surgery. In our experience, FDG-PET appears to be the first line diagnostic method in the detection and staging of colorectal recurrence and differential diagnosis of pancreatic tumour versus chronic pancreatitis.