This paper describes the quantitative estimation of the aortic regurgitation in a man suffering from a stenosis of the aortic valve. Static-pressure measurements obtained from an arrangement of two catheters were used to deduce the time-variation of the pressure gradient in the aorta. From this, the time-variation of the flux of blood, and hence the regurgitation as a percentage of total cardiac output, was calculated. The most widely used method of calculating the arterial flow of blood is that due to Womersley (1957), which is fully discussed in the book by McDonald (1960). This method, in its standard form, models the aorta as a straight rigid tube filled with incompressible fluid. Using this same model, this paper displays certain exact relationships between the effects of inertia and viscosity in pulsatile flow; it is then shown that, if the total cardiac output is used as a governing parameter, the neglect of viscosity introduces analytical errors which, at any rate for the human, are smaller than those other errors associated with the problem. The simplifications thus introduced into the analysis may render the method suitable for routine diagnostic purposes. It is a matter of some regret to the author that he has been able to apply the method to observations from only a single patient; it is hoped that the validity and usefulness of the method will be confirmed by many other sets of data.