OBJECTIVE. Our objectives were to show that an opaque azygos lobe on the anteroposterior chest radiograph can be a normal finding and to investigate the causes of the increased density. MATERIALS AND METHODS. We prospectively examined 53 patients seen during a 3-year period in whom an azygos lobe was detected on chest radiographs. The size of the lobe was determined by the location of the trigone on chest radiographs. The trigone is the triangular area that marks the upper portion of the azygos fissure. If the trigone was located on the lateral aspect of the pulmonary apex, the azygos lobe was classified as type A (nine patients); if the trigone was situated at the midpoint of the cupula of the apex, the lobe was considered type B (20 patients); if the trigone was located on the medial face of the apex, the lobe was considered type C (24 patients). An opaque azygos lobe was defined as an increase of density suggestive of a pulmonary or mediastinal process. The findings on chest radiographs were compared with those on CT scans, which were obtained in all patients. RESULTS. Radiographs showed abnormally opaque azygos lobes in seven patients. Three cases were type B and the opaque lobe had a triangular aspect, suggesting pulmonary disease. Four cases were type C and the opacity had a convex aspect, suggesting a mediastinal mass. In all cases, CT scans showed a shallow azygos lobe and increased depth of the soft tissues of the mediastinum in front of the azygos lobe. This appearance was caused by tortuosity of the supraortic vessels in six cases and by a shadow from a normal thymus in one case. No pathologic process was discovered in any of the patients. CONCLUSION. We believe an opaque azygos lobe can be a normal finding, usually resulting from overlapping tortuous supraortic vessels.