A transseptal left atrial cannulation system which provides drainage of left atrial blood without the need for thoracotomy. A guide wire and a long needle assembly are inserted into a catheter. A cannula rides over the exterior of this catheter. The guide wire may be advanced past the needle assembly and through a catheter through the distal end of the catheter to assist in directing the system to the right atrium. The cannulation system is inserted in a femoral vein located in the groin. Both the guide wire and needle assembly are long enough to allow a substantial length to extend out of the body at the groin for manipulation even when the distal ends of the guide wire and needle assembly are positioned in the heart. When the catheter distal end is positioned adjacent the septum in the right atrium, the guide wire is withdrawn from the catheter orifice and the needle assembly moves past the guide wire and through the catheter orifice to a position adjacent to the septum. The needle pierces the septum and the catheter moves over the needle assembly to further dilate the septal hole. The cannula attached to the catheter also moves through the septal hole, further dilating it, and resting with the holes in the left atrium. The guide wire, the needle assembly, and the catheter are withdrawn from the cannula. Oxygenated blood from the left atrium drains through the cannula to the extracorporeal pump and back to the body through an arterial cannula.