The proximal isovelocity surface area (PISA) method for quantifying valvular regurgitation uses an echocardiographic image with superimposed colour Doppler mapping to visualise the contours of velocity in the blood travelling towards the regurgitant orifice. The flux of blood through the regurgitant orifice is obtained as the product of the area of one of these (presumed hemispherical) contours and the speed of the blood passing through it. However, colour Doppler mapping measures the velocity component towards the echo probe (v cos theta;) rather than speed (v), so that the contours of equal Doppler velocity (isodoppler velocity contours) differ from isospeed contours. We derive the shape of the isodoppler contour surface obtainable by colour Doppler mapping, and show that its area is much less than that of the hemispherical isospeed contour. When regurgitant flux is derived from an appropriate single measure of contour dimension, an appropriate result may be obtained. However, if the true echocardiographic surface area is measured directly, the regurgitant flux will be substantially underestimated. Indeed, the conditions necessary for isodoppler velocity contours to be hemispherical are extraordinary. We should not therefore make deductions from the apparent shape for the convergence zone without considering the principles by which the image is generated. The discrepancy will assume practical significance when increased resolution of colour Doppler technology makes measurement of apparent surface area feasible. Assuming the flow contours are indeed hemispherical, a 'correction' factor of 1.45 would be required.