The diameters of the ascending and descending aorta at the level of the carina were measured from computerised tomograms in 200 adults without cardiac or aortic disease. At all ages the ascending aorta had a greater cross sectional area than the descending aorta, and both areas increased significantly with age. The increase was proportionately greater in the descending than in the ascending aorta and the percentage changes were similar in males and females, the latter having a smaller mean descending aortic diameter. The extent of the increase in cross sectional area of the aorta is sufficient to explain the observed fall of stroke distance that occurs with age. The effect of changing blood pressure on aortic cross sectional area, and hence the relation between stroke distance and stroke volume, was calculated from published data on aortic compliance at different ages. Assuming constant peripheral resistance, stroke distance would change by 34, 82, and 94% for a 100% change of stroke volume at age 20, 50, and 80 respectively. At age 80 the aorta behaves like a rigid pipe but at age 20 its elasticity is such that constancy of aortic size cannot be assumed.