The uptake of fluorine-18 fluorodeoxyglucose (FDG) is increased in processes with enhanced glycolysis, including malignancy. It is this property of FDG which is exploited in positron emission tomography (PET) imaging for lymphoma. FDG, whilst a good oncology tracer, is not perfect and there are limitations to its use. FDG may have low uptake in some types of lymphoma, predominantly low-grade lymphomas. High physiological uptake may occur within the bowel, urinary tract, muscle, salivary glands and lymphoid tissue. FDG is not specific for malignancy and increased uptake occurs in benign conditions with increased glycolysis such as infection, inflammation and granulomatous disease. Benign conditions usually have lower uptake than malignancy but there is overlap. These limitations of FDG mean that tumour may be 'missed', 'masked' or 'mimicked' by other pathology. These limitations are described in this article and methods to circumvent them where possible are discussed. These include performing baseline scans at presentation with lymphoma for comparison with post-treatment scans, simple manoeuvres to reduce physiological uptake such as administration of frusemide and diazepam and remaining alert to the possibility of alternative pathology in immunosuppressed patients. Patients with disease secondary to human immunodeficiency virus are a particular challenge in this regard as they often have dual or multiple pathology. One of the most important skills in PET reporting may be to recognise its limitations and be clear when a definitive answer cannot be given to the referring clinician's question. This may require using PET to direct the clinician to biopsy the site most likely to yield the correct diagnosis.