An improved dilation catheter for use in the conduct of transluminal angioplasty techniques whereby measurements may be taken to determine patency during the course of the procedure. The catheter comprises an elongated tubular member having an inelastic expander member proximate its distal end. The expander is adapted to be inflated and deflated through the introduction of a suitable fluid into the lumen of the tubular member. Disposed on either side of the expander member are sets of electrodes, preferably ring electrodes. Associated with each such electrode is a conductor which runs the length of the catheter from an associated ring electrode to the proximal end of the catheter where such conductors are connected to circuitry for performing impedance plethysmography. Thus, as the improved catheter is routed through the vascular system, the cross-sectional size of the blood vessel at plural points proximate the distal end of the catheter can be assessed, both for locating stenotic lesions to be treated and for later determining the efficacy of the treatment process.