The aim of the present paper was to determine why mitral stenosis is considered critical when the valve area is < or = 1 cm2, and an aortic stenosis when the valve area is < or = 0.8 cm2. An area-resistance curve was drawn for 490 patients with mitral stenosis, and for 525 with aortic stenosis. All points lie on the same curve, but mitral stenosis fell in the low resistance, while aortic stenosis fell in the high resistance section. Critical stenosis respectively begins and ends at the point of curve inflection, i.e. where the valve resistance increases or decreases sharply.