This article describes the use of 18-fluorodeoxyglucose (FDG) scans for the diagnostic evaluation of cancer patients. Since FDG is a positron emitting radio-pharmaceutical, a dedicated Positron Emission Tomography unit (PET) or a hybrid PET/SPECT (single photon emission computed tomography) unit is required for oncologic studies. As PET and PET/SPECT scanners become available to perform these images, it is important that knowledge regarding the usefulness of these images, and their limitations, be disseminated to health care providers. There are evolving uses of FDG scans; current use includes staging patients with non-small cell carcinoma of the lung, malignant melanoma, Hodgkin's disease, non-Hodgkin's lymphoma, colorectal carcinoma, and head and neck carcinoma.