A circumaortic left renal vein simulated a retroperitoneal lymph node metastasis on computed tomography (CT). An intravenous bolus injection of contrast medium followed by dynamic CT scanning and analysis of the attenuation coefficients and the anatomic course revealed the vascular nature of the circumaortic left renal vein. The recognition of this anomaly is important in the differential diagnosis of retroperitoneal lymphadenopathy. A similar analysis should be useful in identifying other retroperitoneal arteries and veins.