Absence of the azygos vein.

A rare case of the absence of the azygos vein (AV) is presented. Imaging findings on plain chest film and on contrast-enhanced spiral computed tomography (CT) are described, and embryology of azygos and hemiazygos veins (HV) is reviewed. On PA chest radiographs, both the absence of AV shadow on its usual location and the presence of the aortic nipple may consider agenesis of AV, and this anomaly can be easily confirmed by CT.