To estimate in patients the accuracy of coronary flow measurements performed by means of 0.014" Doppler wires, the time-averaged maximal blood velocity (APV) was recorded in the 3 branches of 36 angiographically normal coronary artery bifurcations selected in 21 patients undergoing cardiac catheterization for various diseases. Contrast medium injections filmed under two incidences allowed identification of the 3 sample volume locations and computing of the 3 corresponding vessel cross-sectional areas (CSA) at subsequent data analysis. Multiplication of the velocities APV/2 (range: 3 to 20.5 cm/s) by the CSA (obtained by averaging the two calibrated vessel diameters; range: 1.6 to 5.4 mm) yielded 108 flow rates (range: 5.4 to 169 mL/min). The average relative flow error was then estimated using the continuity equation (Q(in) = Q(out,1) + Q(out, 2)) and the central limit theorem. The result was that the relative flow error decreased from 30% at Q = 30 mL/min to 13% at Q = 160 mL/min. We conclude that coronary flow measurements are reasonably accurate, except perhaps for very low flows.