Clinically isolated aortic stenosis is most commonly caused by a congenitally malformed aortic valve. Many elderly patients with isolated aortic stenosis have a tricuspid aortic valve. It has been suggested in the past that inequalities in individual cusp size could be a factor leading to the development of the stenoses in these cases. In this study the width and height of individual aortic valve cusps were measured at necropsy in 200 hearts of patients without clinical signs of aortic stenosis, though the majority of valves obtained from elderly persons showed some ageing changes. The results were compared with similar measurements in 16 hearts of patients with clinically diagnosed and necropsy proven isolated aortic stenosis. The results show that inequality in cusp size is the rule rather than the exception. Indeed, only 5 of the 200 'normal' tricuspid aortic valves had 3 cusps of identical height and width, and each of the 16 cases with isolated aortic stenosis has cusps of differing size. The results seem to contradict the aforementioned hypothesis. However, the striking similarities between isolated aortic stenosis and the ageing aortic valve, suggest that isolated tricuspid aortic stenosis is an extreme with a spectrum of ageing alterations. Inequality in cusp size, with differences in mechanical tissue stress as a consequence, could then play a role in the pathogenesis of the stenosis by accelerating the ageing process.