Aortic valve stenosis is a potentially serious condition. Progression from mild to severe aortic stenosis is well-recognized but there are few data as to the likely rate of progression. Clinical outcome and cardiac catheterization data were reviewed for 65 patients with valvar aortic stenosis. Each patient had been investigated by cardiac catheterization on at least two occasions, the interval between studies ranging between 1 and 17 years (mean 7 years). In 60 cases the aortic valve gradient had increased, from a median of 10 mmHg (range 0-60) to a median of 52 mmHg (range 15-120). The mean rate of increase of gradient was 6.5 mmHg per year, and was significantly faster in patients in whom there was aortic valve calcification or aortic regurgitation present at the first catheter study (P less than 0.02). This study shows that progression of aortic stenosis may be very rapid, and correlates with valve calcification and regurgitation. If cardiac surgery is proposed for co-existing coronary or mitral valve disease in patients with mild or moderate aortic valve gradients, then aortic valve replacement should be considered at that time.