Abstract: The invention provides an apparatus and method for the ablation of cardiac electrically conductive tissue by laser photocoagulation. Reference points of cardiac tissue to be ablated are transvenously mapped. A laser balloon catheter having electrodes positioned on each lateral aspect of the balloon is transvenously positioned in the coronary sinus a the predetermined reference points. The electrodes are used to localize the laser balloon by detecting electrical activity of the tissue adjacent to the laser balloon at the area to be ablated. The tissue area adjacent the balloon is then irradiated to thermally damage the cardiac tissue outside of the vein and adjacent the balloon to render the cardiac tissue electrically inert.