Six patients suspected to have "pacemaker syndrome" were studied with hemodynamic and electrophysiologic methods. Symptoms appeared after implantation of ventricular pacemaker (VVI). Mile symptoms such as dizziness and fatigue and severe symptom of heart failure were noted. The results of the study suggested that cardiac output decreases due to loss of atrial contribution to ventricular filling and also as a result of increase of systemic venous pressure. If the patient has V-A conduction, cardiac output will decrease further. Restoration of atrioventricular synchronization is essential in the treatment of the pacemaker syndrome.