Inferior vena cava agenesis presenting as deep vein thrombosis. Report of one caseAbnormalities of the inferior vena cava are rare. Its embryological development occurs between the sixth and eighth week of gestation and depends on the persistence or regression of three pairs of veins: the posterior cardinal veins, the subcardinal veins and the supracardinal veins. The type of congenital alteration depends on the moment that embryogenesis is altered. The most frequent clinical presentation is deep vein thrombosis, which occurs mainly in young men. We report a 16-year-old male presenting with edema of the left leg. No risk factor for thrombosis was recorded. A Doppler ultrasound confirmed the presence of a deep femoro-popliteal vein thrombosis. An abdominal CAT scan showed an agenesis of the supra and infra-adrenal inferior vena cava, with multiple collaterals. The study for thrombophilia was negative. The patient was treated with vitamin K antagonists with a good clinical response.