Positron emission tomography (PET) is an imaging modality that generates in vivo maps of tissue radioactivity originating from a labelled substrate of glucose metabolism: 18-fluorine labelled deoxy-glucose (FDG). This study was undertaken to evaluate PET in the detection of head and neck malignancies, and to determine its effectiveness in diagnosing recurrent cancer in operated or irradiated fields. PET revealed that each biopsy-proven tumour is an area of increased radioactivity. Tumour radioactivity ranged from 130% to 300% above that of the cerebellum, and up to 650% above the contralateral, normal side. By basing the maps on tissue metabolic function, PET proved capable of distinguishing tumour (increased radioactivity) from scar tissue (reduced radioactivity). Its application may facilitate the diagnosis of recurrent tumours amid the fibrosis and distortion of normal architecture in operated, irradiated fields.