The pacemaker syndrome is an iatrogenic disorder that can result from hemodynamic sequelae of ventricular pacing. Symptoms range from fatigability to syncope and occur during the time the ventricles are being stimulated by the pulse generator. Postulated mechanisms include loss of atrioventricular synchrony, vasodepressor reflexes, and retrograde atrial activation. Prevention is attempted by selection of the appropriate pacing mode for the individual patient. Remission results from restoration of atrioventricular synchrony.