A balloon catheter system generally having an elongated catheter shaft with enhanced flexibility to facilitate more distal advancement within a patient's body lumen. The catheter shaft has a relatively stiff proximal shaft section, a relatively flexible distal shaft section and an intermediate shaft section which provides a smooth, flexible transition between the proximal and distal shaft sections. The intermediate shaft section has an improved construction with a tubular reinforcing member secured by its proximal end to the distal extremity of the proximal shaft section and a distal end secured within the proximal end of the distal shaft section. The tubular reinforcing member preferably has an inner tubular support member which has a proximal end secured to the distal end of the proximal shaft section. Preferably, the proximal shaft section has an outer jacket which extends over the tubular reinforcing member of the intermediate shaft section. Dual lumen and concentric distal shaft sections are describes. Additionally, over-the-wire constructions are described in which the inner and outer tubular members forming part of the distal shaft section extend the length of the catheter.