Abstract: A instrument and method for effecting the dilatation of an artery. The instrument comprises a catheter and an expandable balloon. The catheter includes first and second lumens extending longitudinally therethrough and in communication with respective portions of the balloon. The balloon is an everted member having a cylindrical outer wall and a cylindrical inner wall disposed within the outer wall. The inner wall has a central passageway extending therethrough communicating with the first lumen through which a conventional guide wire may be extended. The inner wall of the balloon is collapsible radially inward, e.g., upon the application of a vacuum to the first lumen, whereupon it frictionally engages the guide wire to prevent relative movement therebetween and to enable the instrument and guide wire to be moved as a unit. The outer wall of the balloon is collapsible radially inward, e.g.