A device in the form of a conversion adapter for reducing an internal lumen of a catheter so as to accommodate a guidewire having a diameter substantially smaller than the internal lumen of the catheter as well as a catheter system including such a conversion adapter is provided. The adapter is positionable within the internal lumen of the catheter. The adapter has an external diameter which is substantially equal to the internal lumen of the catheter and further includes a smaller internal lumen which is substantially equal in diameter to the guidewire and is therefore capable of accommodating the smaller diameter guidewire therethrough. Preferably, the adapter extends within the internal lumen of the catheter along the entire length thereof, and preferably extends beyond the length of the catheter, including a flexible tapered tip at the end portion. In alternate embodiments, the adapter is slidably positionable and removable within the internal lumen of the catheter, thus providing a catheter system which is capable of effectively accommodating guidewires of two different diameters, without creating a drastic transition between the catheter tip and the guidewire and without compromising strength of the delivery system.