Identification of the unknown primary tumours in patients presenting with cerebral metastasis is a continued diagnostic challenge. Despite extensive and lengthy diagnostic work-up, the primary tumours will remain obscure in a significant proportion of the patients. The aim of this study was to evaluate the use of whole-body 18-F-fluorodeoxyglucose positron emission tomography (18FDG PET) scanning in this pursuit. Sixteen patients aged 34-74 years, with histologically confirmed metastatic brain tumours, were included in the study. Whole-body 18FDG PET identified pulmonary foci of probable primary tumours in all patients. Subsequent confirmation of tumour tissue was determined either by direct histological verification or indirectly by the observation of lesion appearance or lesion growth on structural imaging. This could only be obtained in eight of 16 patients, all defined as true positive. Of the remaining eight, a biopsy could not be sampled from seven patients, because of death or limited follow-up investigations, and one patient had pulmonary malignant melanoma metastases. Whole-body 18FDG PET scanning is a sensitive tool in the search for unknown primary tumours of patients with confirmed cerebral metastases allowing early and focused histological confirmation from suspicious lesions.