Abstract: A dilatation balloon having at least one helical groove disposed in at least a portion of the dilating surface of the dilatation balloon is mounted on the distal end of a catheter shaft. Multiple helical grooves may be provided in the dilatation balloon extending parallel to each other or opposite to each other in the dilating surface. Each helical groove has a pitch, width and depth. According to one aspect of the invention, the pitch, width and depth of the helical groove, as well as the number of grooves, are selected to allow perfusion of blood past the dilatation catheter when the dilatation balloon is inflated in a blood vessel. According to another aspect of the invention, the pitch, width, and depth of the helical groove, as well as the number of grooves, are selected so that the dilatation balloon conforms to a tortuous blood vessel and does not straighten the blood vessel during dilation.