Positron emission tomography with fluoro-18-deoxyglucose as tracer molecule (FDG-PET) is a relatively new imaging technique used in oncology to study tumour metabolism in vivo. Both qualitative and quantitative data obtained by PET provide unique information to the clinician and may guide the therapeutic approach in selected patients, where conventional diagnostic tests like CT or MRI yield equivocal results. According to the experience obtained in the Vrije Universiteit Medical Centre in Amsterdam, the additional value of FDG-PET can be explained by the sensitivity and the specificity of the technique, combined with the visualization of the whole body. FDG-PET may reveal metastases and tumour tissue may be differentiated from scar tissue and necrosis. PET is expensive and its effects on patient outcome has yet to be established.