A steerable catheter having a Doppler crystal at the tip to measure the velocity of blood in vivo--and inferentially the blood flow. The catheter includes two passageways, one passageway being exposed to the blood stream with the other passageway containing the electrical leads to the Doppler crystal. The one passageway is dimensioned for receiving a wire guide for accurately manipulating the catheter. Preferably, the catheter incorporates an inflatable angio-balloon operable to distend the blood vessel in the region of a stenosis. Thus, blood flow can be determined in the region of the stenosis before and/or after the vessel is distended. The passageways are preferably defined by a pair of generally concentric tubes with the Doppler crystal doughnut-shaped and sealingly disposed at the tip of the catheter.