An improved guiding catheter (IGC) for PTCA is presented. The IGC's distal tip is formed into a slit. This provides two advantages: 1) Via the proximal portion of the slit, positioned in the sinus of Valsalva, coronary perfusion is assured. With the IGC the flow is twice as great in vitro as with a guiding catheter with two sideholes. Since blood pressure is measured at the point of entry into the coronary artery, coronary flow can be reliably monitored through the pressure curve. 2) Positioned in the left main stem, the guide wire and the balloon catheter can be lead out of the slit in any chosen direction, with the position of the IGC remaining stable and both major branches being well perfused. The IGC was used in the treatment of 90 patients with good results and without complications.