Inferior vena cava (IVC) absence is thought to have either embryologic or developmental etiologies, depending on the degree of absence. Entire absence of the IVC is thought to be due to embryologic insult, whereas recent studies propose that infrarenal IVC absence is developmental, secondary to perinatal thrombosis. Here we report on an adolescent woman with infrarenal absence of IVC and common iliac veins. Clinically, she presented with bilateral lower-extremity numbness and ataxia following strenuous exercise (running > 1 mile). Symptoms resolved with 30 seconds of rest. Radiographically, MRI revealed extensive collateral vasculature that had developed within the paravertebral soft tissues and epidural space of the spinal canal; these collaterals coursed through the neural foramina and caused moderate stenosis at L4/5 and L5/S1.