Despite different aetiologies, acquired aortic stenosis is a self-maintaining, slowly progressive process with good long-term prognosis. In 142 patients with mild stenosis, there was clinical progression within 10 years of the initial diagnosis in only 12% of patients. Twenty-five years after the diagnosis had been established, the severity of aortic stenosis was clinically unchanged in 38%, while 25% of patients had moderate stenosis and 35% had undergone valve replacement. Progression of moderate aortic stenosis was more rapid: the average time interval between the manifestation of moderate aortic stenosis and surgery was 13·4 years. Age at the onset of initial symptoms was related to aetiology: 39±18 years with rheumatic aortic stenoses, 48±6 years in patients with bicuspid valves who had no history of rheumatic fever, infective endocarditis or myocarditis, and 66±12 years in degenerative, calcific stenoses of tricuspid aortic valves. Patients with haemodynamically severe stenosis who had refused the recommended operation (n = 55) had an overall poor prognosis: mean survival averaged 23±5 months and the five-year probability of survival was 18±7%. All these patients died within 12 years of observation. Mean survival after the occurrence of angina pectoris was 45±13 months, after syncope 27±15 months, and after first occurrence of left heart failure 11±10 months.