facilitate the withdrawal of the original catheter 10 and the introduction and advancement of the replacement catheter with a new distal shaft section through the patient 39 s arterial system until the more appropriately sized replacement balloon extends across the stenosis the replacement balloon may then be inflated one or more times in a conventional manner to dilate the stenotic region of the patient 39 s artery and then be withdrawn as the original catheter 10 fig3 illustrates a rapid exchange type dilatation catheter 40 embodying features of the invention which has a proximal shaft section 41 a distal shaft section 42 a dilatation balloon 43 on the distal shaft section and an adaptor 44 on the proximal end of the proximal shaft section the proximal shaft section 41 is preferably hypotubing formed of metal such as stainless steel e g 304 or pseudoelastic niti alloy provided with male threads 46 which are configured to threadably engage the female threads 47 on connector element 48 secured to the proximal end of distal shaft section 42 as shown in fig3 the distal shaft section 42 is provided with a guidewire receiving inner lumen 50 which extends from proximal guidewire port 51 to the distal guidewire port 52 provided in the distal end of the catheter a dual lumen portion 53 extends from the connector element 48 to.