An improved coronary sinus catheter has been developed whose enhanced flexibility facilitates catheterization of the great cardiac vein. It allows simultaneous thermodilution measurement of blood flow in the great cardiac vein and proximal coronary sinus, easy withdrawal of blood from both sites and bipolar atrial pacing from the proximal coronary sinus. The accuracy of flow measurement with this catheter has been validated by in vitro testing, and the catheter has been used successfully in eight patients. The advantages of this improved catheter over the existing coronary sinus thermodilution catheter are discussed.