A rapid-exchange balloon catheter for medical treatment of a patient includes a shaft with proximal and distal sections. The proximal shaft section has a hypotube with a proximal tubular portion, an intermediate tubular portion with a longitudinal indentation, and a distal portion. The distal shaft section has an inner and outer tubular body, and their proximal ends are sealed to the hypotube near a transition between the hypotube indented portion and the hypotube distal portion. The inner body defines a guidewire lumen extending from a proximal guidewire port at its proximal end and a distal guidewire port at its distal end. The hypotube distal portion extends into the space between the inner and outer tubular bodies, and provides a flexibility transition from the proximal shaft section to the distal shaft section.