This report describes a patient with drug refractory severe chronic congestive heart failure, first-degree AV block, and wide QRS complexes of 175-ms duration with left bundle branch block who was successfully treated by a left-sided dual chamber pacemaker system using a single-lead coronary sinus electrode. Left atrial and ventricular signal amplitudes, left atrial and ventricular pacing thresholds were 2.5 mV, 13 mV, 1.8 V and 0.7 V (0.5 ms), respectively. Using DDD pacing with an optimal 120-ms AV delay the QRS duration decreased to 150 ms and the mitral filling pattern improved. Three months after implantation the patient's functional status improved from NYHA Class III-IV to Class II.