The etiology of acquired aortic stenosis (AS) has changed dramatically as socioeconomic and hygienic conditions have improved and as the general population lives to an older age. Rheumatic disease was responsible for most cases of AS until a few decades ago, whereas now most are due to calcific degenerative or bicuspid etiologies. There is a long latency period from the initial discovery of a murmur and first onset of symptoms. In studies representing clinical experience prior to the 1960s, the mean age at symptomatic presentation was 48 years, while in series representing experience up the 1980s, it was 61 years. The changing etiology of AS has important implications for following patients with AS, and monitoring those who are discovered to have significant AS in the absence of symptoms. AS has become more a disease of the elderly, and it is the elderly patient with AS, especially those with calcific degenerative AS, who develop the most rapid and significant progression of their disease, present with symptoms of left ventricular (LV) failure, and are most likely to have critical outflow tract obstruction at the time of their presentation. Once symptoms develop, the outcome of patients with AS is quite poor: in early studies approximately 50% of such patients were dead at 5 years and 90% were dead at 10 years. Symptoms that represent LV failure, e.g., dyspnea, are associated with a worse survival (average survival 2 years) compared to symptoms that represent LV hypertrophy, e.g., angina or outflow obstruction, syncope (average survival 3 years). There is uniform agreement that once symptoms develop, patients with significant AS should undergo valve replacement.(ABSTRACT TRUNCATED AT 250 WORDS)