18F-FDG PET scan is an useful functional whole body imaging modality that images various types of malignancies with relative high sensitivity and specificity in a reasonably rapid time. It depicts a lesion based on abnormal glucose metabolism whereas CT detects malignant process mostly based on altered anatomy. In patients with gastric cancers, PET scan detects only less than 50% of early cancers and 62-98% of advanced cancers. For initial T staging, anatomical imaging with a high spatial resolution is essential. There are a few studies on the prognostic significance of FDG uptake with inconsistent results. In spite of low sensitivity for lymph node staging, the specificity of CT and PET scan are very high, and the specificity of PET scan tends to be higher than that of CT. Detection of distant metastases on PET scan is dependent on tumor histology, degree of FDG uptake in primary tumors, sites of distant metastases, etc. There are only a few data available for the evaluation of recurrence detection and treatment responses using FDG PET scan. FDG PET scan has been used in the preoperative staging of colorectal cancer with some promising results. It seems to be the most useful in restaging recurrent tumors and selecting those patients who would benefit from surgery. PET scan has a potential value in assessing treatment responses after various combination of treatments in patients with colorectal cancer.