The gross surgical pathologic features of the aortic valve were reviewed in 374 patients who had had clinically pure aortic stenosis and aortic valve replacement at our institution during the years 1965, 1970, 1975, and 1980. The most common cause of aortic stenosis, accounting for 46% of our cases, was calcification of a congenitally bicuspid valve. In the remainder, stenosis was produced by postinflammatory fibrocalcific disease (including rheumatic disease) in 35%, by degenerative calcification of an aging valve in 10%, and by calcification of a congenitally unicommissural valve in 6%. The cause of aortic stenosis was indeterminate in 4%. Valvular lesions included various degrees of dystrophic calcification, commissural fusion, and cuspid fibrosis. Calcification tended to occur more extensively and at a younger age in men than in women. Furthermore, it tended to produce stenosis and to necessitate valve replacement earliest in patients with unicommissural valves (mean age, 48 years), later in those with bicuspid or postinflammatory valves (mean age, 59 and 60 years, respectively), and latest in those with degenerative stenosis (mean age, 72 years). In our study, the relative incidence of postinflammatory aortic stenosis remained unchanged from 1965 to 1980, despite the steadily decreasing incidence of acute rheumatic fever reported in western countries. Our data suggest that (1) the incidence of chronic rheumatic heart disease has not yet begun to decrease appreciably, (2) many episodes of acute rheumatic fever may be subclinical, or (3) some forms of nonrheumatic aortic valve disease may produce gross alterations indistinguishable from those of classic chronic rheumatic valvulitis.