Aneurysms of the inferior vena cava are rare, with 54 cases reported in the literature. They carry a significant morbidity and mortality risk. A case of an inferior vena cava aneurysm in a patient with Klippel-Trenaunay Syndrome is reported. Open aneurysmorrhaphy of the Type III aneurysm was successfully performed. The patient's leg swelling, back pain and exercise tolerance improved. Inferior vena cava aneurysms are not known to be associated with Klippel-Trenaunay Syndrome. However, clinicians should have a high index of suspicion for great vessel aneurysms in these patients as they are associated with greater thromboembolic risk.