The possibility of determining the real size of the aortic annulus on the basis of adequate angiocardiographic measurements has been retrospectively verified in a group of 10 patients who underwent aortic valve replacement. A good correlation (r = 0.99) was found between the predicted diameter of the aortic annulus and the size of the valve inserted at operation. The useful implications associated with prediction of the diameter of the aortic annulus are discussed.