The mechanical properties of the abdominal aorta were investigated non‐invasively in 30 patients with aortic anaeurysm and II with peripheral arterial disease. The distensibility of the aorta was measured using M‐mode ultrasonography, permitting non‐invasive assessment of the pressure‐strain elastic modulus or aortic stiffness, Ep. The median Ep value increased, from 4.0 N/cm2 in control subjects in their third decade of life ( n = 10) to 10.4 N/cm2 in middle age ( n = 11) to 14.0 N/cm2 in the elderly (n = 13). In the presence of a normal diameter.peripheral arterial disease with aortic atherosclerosis had little effect on aortic stiffness, median Ep being 16.0 N/cm2. Aneurysmal dilatation was associated with a significant increase in aortic stiffness, median Ep being 31.3 N/cm2 ( P < 0.001). For aortas of normal diameter, Ep was at all ages dependent on mean arterial pressure. In patients with aortic aneurysms there was no clear relationship between Ep and mean arterial pressure or aortic diameter. Of the patients studied, 15 underwent aortic rcconstruction; increasing aortic stiffness (log Ep) was associated with a decreased medial elastin content of the aortic biopsy ( r = −0.63, P < 0.02 ). This study demonstrates the marked stiffness or inelasticity of dilated or aneurysmal vessels, part of which is attributable to the loss of elastin.