Patients presenting with undiagnosed fever generally fall into two broad groups: firstly, those with co-existing disease or recent surgery, and secondly those with pyrexia but who are otherwise more or less well. A pyogenic cause for fever is significantly more likely in the first group as compared with the second. Radionuclide agents which are currently freely available for investigating undiagnosed fever are labelled leukocytes, 67Ga, and radiolabelled human immunoglobulin (HIG). Labelled leukocytes are generally preferable in the group with co-existing disease (occult infection), whereas 67Ga should be used in the second group, which can appropriately be called fever of unknown origin (FUO). The lower specificity of 67G can be perceived as an advantage in this context in view of the wide range of pathologies capable of causing FUO. The role of HIG in undiagnosed fever is unsettled. In general, 67Ga is more helpful than labelled leukocytes when the cause of an FUO is intrathoracic, although the reverse is likely to be true for intra-abdominal causes, partly because of physiological excretion of 67Ga in the gut. Postoperative fever is an indication for a leukocyte scan. Patients with haematological malignancies occasionally present as an FUO, but patients with chronic renal disease should be approached as occult infection. Investigating children with undiagnosed fever is a particularly difficult problem. For the future, we need agents which are particularly effective for localising chronic inflammation and, of secondary importance, agents able to distinguish between infective and non-infective causes of inflammation.