Aneurysms of the inferior vena cava (IVC) are extremely rare, with a range of reported presentations including deep venous thrombosis. Computerised Tomography (CT) and Magnetic Resonance Imaging (MRI) scans are suggested appropriate diagnostic imaging modalities, but even then it may not be possible to differentiate between an IVC aneurysm and a retroperitoneal tumour. This case illustrates that an IVC aneurysm should be considered in the differential diagnosis of a retroperitoneal tumour.