A catheter has an electrode assembly adapted for use on an ostium and a stabilizing assembly movable between a stowed position within the catheter and a deployed position in the tubular region of the ostium. The stabilizing assembly has a balloon member that can be extended distally into the tubular region and inflated to wedge itself in the tubular region for anchoring the electrode assembly. The balloon member is mounted on an elongated extender that extends through the catheter and is provided with longitudinal movement to advance and withdraw the balloon between the stowed and deployed positions.
