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Background: Little is known regarding the long-term performance of coronary sinus (CS) leads, which have an effect on the longevity of cardiac resynchronization therapy (CRT) systems. Methods: This study included 109 patients (79 men) whose mean age was 68 ± 9 years, New York Heart Association (NYHA) functional class 3.2 ± 0.5, and left ventricular ejection fraction 25.6 ± 6.6%, and who underwent CRT (n = 45) or CRT-D (n = 64) systems implants for management of idiopathic (53%), ischemic (40%), or miscellaneous (7%) dilated cardiomyopathy. Unipolar (n = 57) or bipolar (n = 52) leads were placed into the CS venous system. Results: At implant, no significant difference was observed between unipolar and bipolar leads with respect to mean sensing performance (14 ± 6 mV vs 14 ± 8 mV, P = 0.97), pacing impedance (875 ± 234 ohms vs 943 ± 331 ohms, P = 0.24), and stimulation energy threshold (2 ± 3.2 μJ vs 1.13 ± 1.5 μJ, P = 0.08). At a median follow-up of 33 months, a significant decrease in stimulation impedance and increase in stimulation energy threshold was observed with unipolar (689 ± 122 vs 875 ± 234 ohms, P < 0.01, and 8.34 ± 10.4 μJ vs 2 ± 3.2 μJ, P < 0.001, respectively) as well as with bipolar (735 ± 268 ohms vs 943 ± 331 ohms, P < 0.01, and 4.81 ± 9.92 vs 1.13 ± 1.5 μJ, P = 0.02, respectively) leads. No significant difference in sensing performance was observed with either type of lead (10 ± 5 mV vs 14 ± 6 mV and 10 ± 6 mV vs 14 ± 8 mV, respectively). At long-term follow-up, no significant difference among any sensing or stimulation parameter was observed between unipolar and bipolar leads. Conclusions: At long-term follow-up, a significant increase in the energy required for stimulation was observed, whereas sensing performance remained unchanged. The increase in energy capture threshold was less marked with bipolar than with unipolar leads. [ABSTRACT FROM AUTHOR]