The authors observed the postaortic left innominate vein, which passed behind the ascending aorta, in the body of a 69-year-old Japanese male during dissection in 1979. In this case, no rudiment of the ordinary left innominate vein could be found in front of the arteries arising from the arch of the aorta. There are few reports of this anomaly and an extensive search of the available literature revealed only ten cases, including three in Japan. Adachi classified the postaortic left innominate vein into two types, types I and II, on the basis of the positional relation between the left innominate vein and the ligamentum arteriosum. Our case was of type I, in which the left innominate vein passed ventral to the ligamentum arteriosum and ran across from left to right behind the ascending aorta to join the right innominate vein. The embryological explanation which generally is accepted for the postaortic left innominate vein lies in the transposition of a transverse connection, which becomes the left innominate vein in a later stage of development, between the left and right anterior cardinal veins from the ventral to dorsal sides of the rudiment of the ascending aorta.