When ring segments of artery free of external loads are cut longitudinally, they spring open. This property which has been termed residual strain, is a manifestation of an underlying residual stress and may be quantified by measuring the degree to which the cut ring springs open. Residual stress serves to redistribute the forces in the media and to minimize the stress gradient between its inner and outer margins. Its magnitude and distribution are closely related to vascular growth and development. We have measured the degree of opening (in terms of an opening angle, see below for its definition) in 168 rings removed from six positions along aortae obtained at autopsy from 30 subjects aged between 3 months and 87 years. At all ages, there was an overall increase in opening angle along the aorta away from the heart. At all positions, the opening angle increased with age and was higher in vessels with visible atheroma. At all ages, the opening angle was significantly greater in males than in females. This difference persisted when allowance was made for the greater incidence of atheroma in males. We have found a similar age-related increase in rats, although there was no difference between males and females. These observations are consistent with a recently proposed 'stress-growth' law in which remodelling of the arterial wall is driven by the non-uniform distribution of stress across it and may be associated with known sex-related differences in the aetiology of cardiovascular disease.