A catheter apparatus captures thromboembolic material in the arterial and venous circulation. The apparatus includes an inner hollow tube and an outer hollow tube in sliding engagement with the inner hollow tube. A radially expandable segment, such as an inflatable balloon, is attached near the distal end of the catheter. One end portion of the balloon is attached to the inner tube and the other end portion is attached to the outer tube. When the catheter is advanced through a vessel to be treated, the balloon is deflated and the outer tube is slidably advanced to a storage position so that the balloon compactly surrounds the catheter. When the catheter is in place within the vessel, the outer tube is slidably advanced to a treatment position and the balloon is inflated. As the vessel is treated, the inflated balloon captures and contains thromboembolic particles that may be released, and the particles are suctioned out of the patient through apertures located in the outer hollow tube proximal to the balloon.