In the present study, we propose a classification of the different forms of the circumaortic renal collar based on the findings of 319 patients and on a literature review. The patients underwent a CT angiography scan of the abdomen for various diagnostic reasons. The different forms in which the circumaortic renal collar may appear were categorized. The renal collar was observed in eight of the 319 (2.5%) patients. The classification we propose is the following: Type I, one left renal vein splitting into two branches, a preaortic and a retroaortic, draining into the inferior vena cava; Type II, two different left renal veins, the one preaortic and the other retroaortic, draining into the inferior vena cava; and Type III, either anastomoses between the preaortic and retroaortic vein, being multiple or not, or multiple preaortic or retroaortic renal veins without anastomoses. Although the circumaortic renal collar is usually asymptomatic, the knowledge of this variation is useful, mainly before abdominal aortic aneurysm surgery and renal transplantation. Failure to recognize the collar's retroaortic component may lead to hemorrhage and death during retroperitoneal surgery. We believe that our classification contributes to recognize the circumaortic renal collar's morphology both preoperatively and intraoperatively.