Positron emission Tomography (PET) with 2-[(18)F]-fluoro-2-deoxy-d-glucose (FDG) is a functional imaging technique with increasing value in special diagnostic fields of gastrointestinal tumours. In the initial staging of esophageal and gastric cancer, FDG-PET is useful in the staging of patients with advanced but local resectable disease. The detection of distant metastases results in an up-staging, and these patients should not be treated by surgery. Furthermore, FDG-PET is sufficient for monitoring early therapy responses after neoadjuvant treatment and enables one to select non-responders who may benefit from therapy alterations. Major indications for FDG-PET in patients with rectal carcinoma are therapy monitoring and diagnosis of relapses, especially the differentiation between tumour and scar and also the localisation of tumour manifestations in cases with increasing tumour markers. FDG-PET is very efficient in the imaging of pulmonal and hepatic metastases of colorectal cancer but not in lymph node staging. In diagnostic procedures for pancreatic carcinoma, FDG-PET can be recommended to explore the dignity of pancreatic lesions and in the imaging of tumour relapses. For gastrointestinal stroma tumours, FDG-PET is useful for the monitoring of therapy and the initial staging. For imaging of hepatocellular carcinoma and carcinoma of the gall bladder, FDG-PET is not sufficient.