Objective : Atrial pacing, despite its beneficial hemodynamic and antiarrhythmic effects, is still underestimated as a mode of choice for the treatment of patients with sick sinus syndrome (SSS). Main purposes of this study were to evaluate long-term outcomes of the pacing modes on SSS and the electrical stability of the atrial myocardium. Patients : One hundred thirty-three patients with SSS treated with pacemaker therapy at Juntendo University Hospital between 1980 and 1995 were followed for a median period of 74 months. Sixty-four patients underwent atrial pacing mode with AAI pacemaker, and 69 did ventricular pacing mode with a VVI pacemaker. Atrial pacing was performed using a lead of active fixation type. Methods : As pacing parameters of the atrium, pacing threshold, P-wave amplitude and impedance were assessed. These parameters were compared between 64 SSS patients with AAI pacemakers and 109 atrioventricular (AV) block patients with DDD pacemakers. Measurement and Results : The intrinsic P-wave amplitude and pacing threshold were found to be constant during the long term follow-up period. No SSS patients with atrial pacing developed advanced AV block. Six (9%) of 64 SSS patients with atrial pacing and 22 (32%) of 69 SSS patients with ventricular pacing developed atrial fibrillation. Electrophysiological characteristics of the group complicated with chronic atrial fibrillation revealed slight prolongation of the atrial refractory period. The pacing parameters of the atrium showed no difference between 64 SSS patients with atrial pacing mode and 109 AV block patients with DDD pacing mode. Conclusions : Atrial pacing is proved to be a safe and reliable treatment for SSS.