A preduodenal portal vein is a rare vascular anomaly that is the result of a variation in the normal developmental pattern of the embryonic precursors of the portal vein, the right and left vitelline veins and their three anastomotic channels. Such an anomalous vein is a hazard in operations on the duodenum or biliary tract because of the dangers of inadvertent division or ligation. The anteriorly placed portal vein may also interfere with the customary placement of anastomosis in instances of congenital duodenal obstruction. There is a remarkably frequent association of preduodenal portal vein with situs inversus of varying degrees and with malrotation.