High pulse pressure (PP) contributes to the pathogenesis of hypertension and is associated with adverse cardiovascular disease outcomes. There is consensus that aortic wall stiffening contributes to higher PP. However, the role of lumen size in the pathogenesis of elevated PP remains controversial. Prior studies showing an unexpected inverse association between PP and lumen area have been criticized for using echocardiography, which affords limited views of the thoracic aorta. Therefore, we performed cine magnetic resonance imaging (MRI) of the ascending aorta 5 mm above the aortic valve and the proximal and distal descending thoracic aorta in 423 older participants (age 72 to 94, mean 79 years; 57% women) in the Age, Gene/Environment Susceptibility-Reykjavik Study (AGES-Reykjavik). Immediately prior to MRI, supine auscultatory blood pressure (141±19/64±9, PP=77±18 mmHg) and tonometry of brachial, radial, femoral and carotid arteries were performed. Mean aortic lumen area during the cardiac cycle was computed at each level and averaged across the 3 levels to give average lumen area (LA_M). Wall area (WA) and elastance (E = PP x A_D/(A_S-A_D)) were similarly averaged. In linear regression models, LA_M was negatively related to PP when considered alone (Model 1) and in a model that adjusted for age, sex, height, weight, heart rate, total and HDL cholesterol, triglycerides, estimated GFR, diabetes, glucose, HbA1c and history of smoking (Model 2). The relation persisted after further adjustment for E and WA (Model 3). In our sample of older people, higher pulse pressure is associated with a smaller lumen area of the thoracic aorta.