Introduction: Aneurysms are defined as an abnormal dilation of an artery, vein or cardiac chamber. Aneurysms affecting the inferior vena cava (IVC) are rare, with just over 50 cases in the published literature. They are associated with caval thrombosis. We will discuss the aetiology and management of such cases. Case description: A 14-year-old girl presented to her local hospital complaining of a two week history of worsening back pain, swelling and discolouration of the legs, and reduced mobility. Imaging suggested a psoas abscess, and drainage was arranged at a regional paediatric centre. Upon review, repeat imaging was sought which indicated an IVC aneurysm rather than a psoas abscess. There was thrombosis within the dilatation extending to the femoral veins which accounted for her symptoms. Results and conclusion: The patient was anticoagulated in the first instance. The duration of the symptoms meant it was too late for thrombolysis, and the occluded segment was considered too long for conventional venous stenting. She has been placed in compression hosiery and referred to the national centre for ongoing management. Take-home message: Unusual presentations are often caused by rare pathologies. In any patient presenting with bilateral swollen, purple legs; it is imperative to establish if there is any venous occlusion. This was also an important lesson in being wary of draining supposed psoas abscesses in young patients.