Among 646 patients with pure aortic stenosis who underwent valve replacement at our institution between 1981 and 1985, the three most frequent causes were calcification of congenitally bicuspid aortic valves (38%), degenerative (senile) calcification of tricuspid aortic valves (33%), and postinflammatory (presumably rheumatic) calcification and fibrosis (24%). Among the 324 patients younger than 70 years of age, calcified bicuspid valves were observed in 50%. In contrast, among 322 patients 70 years of age or older, degenerative calcification accounted for 48% of the stenotic aortic valves. During the 5 years of the study, the relative frequency of postinflammatory disease decreased from 30% to 18%, and that of bicuspid valves decreased from 37% to 33%. In contrast, the relative frequency of degenerative calcification increased from 30% to 46%. Consequently, degenerative (senile) calcification is currently the most common cause of aortic stenosis among patients undergoing valve replacement at our institution. This finding may be related to changes in life expectancy in the general population, alterations in patient referral practices, and an increased willingness of surgeons to operate on older patients. Regardless of cause, the observed temporal changes in etiologic factors for aortic stenosis may indicate a potential source of increasing health-care costs among the elderly population.