The subject guide catheter extension system with a micro-catheter delivery catheter includes an outer sheath, an inner member extending within the sheath, and a mechanism for engagement/disengagement of the inner member to/from the sheath. Several mechanisms of engagement/disengagement between the inner and outer members are provided including a friction mechanism, threaded mechanism, pull away sheath, and engagement/disengagement mechanism for pusher's handles. The sheath and the inner member are modified for different engagement/disengagement mechanisms operation. A micro-catheter delivery system provides for an improved atraumatic crossability to the treatment site in an expedited and simplified fashion. During a procedure, a guidewire along with a guide catheter are advanced to the vicinity of the treatment site within a blood vessel. Subsequent thereto, the subject guide catheter extension system is manipulated to advance the micro-catheter along the guidewire inside the guide catheter towards and beyond the site of interest. Once the micro-catheter is in place, the outer sheath slides along the micro-catheter until reaching the lesion, and then the inner member is removed from the sheath, and the sheath then is ready for passing the treatment catheter (stent/balloon) towards the lesion to be treated.
