Background: Earlier reports suggest that the stimulation site in the left ventricle plays an important role in the outcome of cardiac resynchronization therapy. Although data suggest that the posterolateral (PL) location is ideal, this is not always feasible due to variations in the coronary sinus (CS) anatomy, suboptimal pacing thresholds and diaphragmatic stimulation. The purpose of this study is to assess the relationship between changes in ejection fraction (EF) and pulmonary artery pressure (PAP) and CS lead position.