A 39-year-old man in good health presented to our hospital with acute swelling of the left leg. No risk factors for deep venous thrombosis (DVT) were present, in particular, no immobilization, surgery, known thrombophilia, or family history. Venous duplex ultrasonography demonstrated complete thrombosis of the left iliac and femoral veins, and the inferior vena cava was not detectable. Abdominal magnetic resonance angiography (Cover) confirmed the absence of the infrarenal portion of the vena cava. The renal veins (R) drained into dilated lumbar veins (L), and only at the intrahepatic level was the diameter of the vena cava inferior normal (A). The hemiazygos vein (H) was dilated as a compensatory mechanism. and as anatomically expected, the thoracic dilated hemiazygos system drained into the dilated azygos vein (Az) (B). No congenital defects of the heart or abdominal organs were seen. Screening for thrombophilia was performed without any pathologic findings. The patient was initially treated with low-molecular-weight heparin, followed by warfarin for 1 year and compression therapy. The diagram (C) shows the different pathways of venous drainage from the lower half of the body in absence of the inferior vena cava: through the (1) superficial epigastric vein-thoracoabdominal vein-subclavian vein, the (2) inferior epigastric vein-superior epigastric vein-subclavian vein, and the (3) ascending lumbar vein-azygos vein-superior vena cava. In 80% of patients presenting with DVT, a risk factor can be identified. 1 An absent or hypoplastic infrarenal vena cava is a rare risk factor for DVT in young adults. The drainage of the lower limbs through the azygos vein might be insufficient, causing stasis and eventually thrombosis. 2 Venous claudication might be present. The prevalence of congenital absence of the vena cava inferior is estimated at 0.5% of the general population. 3