A catheter system for localized delivery of a therapeutic or diagnostic agent within a vessel is provided. The system provides for adjustability of the length of the treatment area and for reducing of pressure within the treatment area. The catheter system includes an inner elongated element, an outer elongated element positioned coaxially with respect to the inner elongated element, a blood-release element at a distal end of the inner elongated element and a supply elongated element coaxial to the outer elongated element. A proximal occlusion element is positioned at the distal end of the outer elongated element, proximal to an outlet port. A distal occlusion element is positioned at a distal end of the inner elongated element. The distal end of the inner elongated element is distal to and movable with respect to the outer elongated element distal end.
