Abstract: A catheter is attached to an elongated catheter body adapted for endovascular insertion with a balloon assembly at its distal end. Coolant injected through the catheter body may, in different embodiments, directly cool tissue contacting the balloon, or may cool a separate internal chamber. In the first case, the coolant also inflates the balloon, and spent coolant is returned to the handle via a return passage extending through the body of the catheter. A valve may regulate back pressure in the return passage to coordinate the flow of coolant into and out of the balloon so as to both inflate the balloon and achieve cryogenic cooling at the surface of the balloon. The coolant is biologically safe, and may be liquid carbon dioxide. Plural balloons may be provided adjacent the cooling segment, and one balloon may be shaped to treat the ostium of a vessel.