Positron emission tomography (PET) is an imaging technique using biological tracers (18-fluorine fluorodeoxyglucose (FDG)), whose uptake into tumour cells is increased. Previous studies carried out in patients with lymphoma have shown that PET is an accurate method of staging disease and that a high pre-treatment FDG uptake that is abolished by the end of chemotherapy is associated with a good clinical response. This case report demonstrates that clinical PET scanning can be especially useful in staging patients with lymphoma in whom there is extensive extranodal and marrow involvement, and that PET can follow changes in tumour viability over a very short time span.