The utility of azygos vein measurement in determining pathologic processes is discussed on the basis of reports in the literature and normal values obtained from 762 chest radiographs of patients from birth to fourteen years of age. Four age groups were defined and mean vein size obtained: birth to 6 mo., mean 3.5 ± 1.3 mm; 6 to 24 mo., mean 4.1 ± 1.0 mm; 2 to 7 yr., mean 4.8 ± 1.2 mm; 8 to 14 yr., mean 5.1 ± 1.6 mm. The azygos vein does not increase uniformly in size with age. Respiratory activity, posture, and anatomic arrangement may cause changes in azygos vein width from subject to subject, and from time to time in the same subject, making c1inicoradiologic correlation uncertain.