Accepted clinical views about the natural history of aortic stenosis are based on surprisingly little hemodynamically documented data, and further information in unlikely to be forthcoming in the modern surgical era. Therefore, follow-up data were obtained on 42 adult patients with isolated valvular aortic stenosis catheterized at Georgetown University Hospital who did not undergo early valve replacement. Of 32 symptomatic patients, 23 had moderate or severe stenosis and were followed until death or for an average of 64.4 months after catheterization. The prognosis was more ominous than previously reported. Mortality rates from onset of symptoms were 26% at one year, 48% at two years, and 57% at three years. Fifty-six % of deaths occurred suddenly, within hours of new symptoms. Asymptomatic patients with moderate or severe stenosis did not share the high mortality rate of those with symptoms. Eight such patients were followed for an average of 69.8 months, and none died.