The absence of the inferior vena cava is an uncommon congenital anomaly and can be misdiagnosed. We report a 14-year-old boy initially admitted as a result of a painful abdominal mass; after admission, he experienced a deep venous thrombosis in his left leg. Subsequent evaluation revealed the congenital absence of the entire inferior vena cava, with a cluster of thrombosed collateral veins in his right pelvis, corresponding to the abdominal mass. The recognition of this anomaly may be helpful in the event of differential diagnosis in retroperitoneal masses. In young patients with idiopathic deep venous thrombosis, an inferior vena cava anomaly should be considered.