Anomalies of the inferior vena cava (IVC) have been recognized as one of the predisposing factors for deep vein thrombosis. Rarely, thrombosis of an anomalous retroperitoneal vein may resemble a soft tissue mass. Awareness of this fact helps preventing unnecessary interventions. We report a case of thrombosis of retroaortic left renal vein and interrupted IVC that mimicked a retroperitoneal neoplasm.