The mediastinal structures said to lie on a horizontal plane at the level of the manubriosternal joint (manubriosternal plane) in the cadaver include the bifurcation of the trachea, the concavity of the arch of the aorta, and the azygos vein as it arches over the right principal bronchus to enter the superior vena cava. We have reviewed CT scans of the thorax in 51 subjects to determine 1) whether these structures lie consistently at this level in the living thorax and 2) the vertebral level of this plane. We found that the bifurcation of the trachea lay at the plane in 41% of subjects, that the plane passed through the concavity of the arch of the aorta in 49% of subjects, and that, although there was notable individual variation, the manubriosternal plane passed through the upper part of the fifth thoracic vertebra in 53% of cases.