PET/CT has proven to be extremely useful in studying neoplasms of the colon and esophagus. It has been less promising for lesions of the stomach, pancreas and hepatobiliary tract. Colorectal cancer is the third most common non-cutaneous cancer representing 13% of all malignancies. The use of colonoscopy has significantly contributed to the earlier detection and higher cure rate. PET/CT is not a screening procedure. It is very good for staging, recurrence detection and monitoring therapeutic interventions. It is excellent for detecting distant metastases, e.g. liver lesions, but is less accurate for detecting nodal involvement. The CT portion of the study enhances certainty of lesion localization and characterization. Esophageal cancer is less common in the U.S. in that it represents 7% of G-I cancers, but only 1% of all cancers. The major problem is that often it is advanced to Stages III or IV before it comes to clinical recognition. A 5-year survival has been improved from 3% to 10% by the use of induction chemoradiotherapy. PET has proven useful in staging and determining resectability, monitoring response to therapy, radiotherapy treatment planning and distinguishing between postop scar and residual or recurrent disease on CT. Gastric cancer results have been more variable. The intestinal (tubular variety) shows better uptake than the non-intestinal (signet ring cell) variety because of the greater mucous content of the latter which is associated with more false negatives. FDG uptake in pancreatic cancer is also variable. Attempts at distinguishing carcinoma from pancreatitis have been limited. When lesions do show uptake, PET/CT has been helpful in monitoring therapeutic interventions. Hepatocellular cancer demonstrates significant FDG uptake in only 50-70% of cases. Cholangio carcinomas; particularly the peripheral variety, do show significant FDG uptake.