The capability of a pulsed Doppler flowmeter combined with a phased-array imaging system to measure volume flow was tested in vitro and in patients undergoing cardiac catheterization. The Doppler-phased-array system (DPA) was used to determine vessel diameter and a superimposed cursor was used to locate the range and angle of the Doppler sample volume. DPA estimates of continuous flow through tubing in a water tank correlated strongly (r = 0.99) with measured flow corresponding to physiologic ranges from 3-12 l/min. For pulsatile flow in a water tank, a correlation of r = 0.86 with measured flow was obtained, whereas DPA estimates of cardiac outputs compared with Fick estimates in the 11 patients produced a correlation of r = 0.83. These data indicate that estimates of cardiac output are possible using the DPA approach.