Tumour thrombus is a rare complication of many solid cancers including renal cell carcinoma, Wilms' tumour, testicular tumour, adrenal cortical carcinoma, lymphoma, pancreatic cancer, osteosarcoma and Ewing's sarcoma. We describe six patients who harboured occult tumour thrombus detected by fluorine-18 fluorodeoxyglucose (F-FDG) positron emission tomography (PET)/X-ray computerized tomography (CT) imaging as part of the staging investigations. Recognition of this rare complication by PET/CT can change the management plan and prevent unnecessary long-term anti-coagulation treatment because of wrong diagnosis of cancer-related venous thrombus.