Fifty patients, aged 23 to 88 years, with permanent rate-responsive dual chamber pacemakers were studied prospectively for 14.1 +/- 11.4 (S.D.) months after implantation to assess the benefits and complications associated with this technique. In 12 patients the device replaced a ventricular demand pacemaker. Minor complications associated with implantation occurred in one case. Atrial leads required repositioning because of increase in threshold and/or problems of sensing in five cases and ventricular leads in five. There were two patients with symptomatic pacemaker-related arrhythmias necessitating reprogramming; one patient with pacemaker-mediated tachycardia and one with pacemaker autoinhibition. Seven patients have died; one suddenly and possibly related to a pacemaker-triggered arrhythmia. Of 43 living patients, five are now programmed to the ventricular demand mode; two with atrial fibrillation, one with failed atrial lead repositioning, one with persistent sinus tachycardia, and one because of angina pectoris. Thirty-six of the 43 living patients are asymptomatic and a further six are symptomatically improved. All 12 patients changed from ventricular demand pacing have less symptoms. Rate-responsive dual chamber pacing is safe and appears to improve symptoms in most cases. Complications are infrequent and usually easily overcome. This mode of pacing should be considered in all patients with normal sinoatrial function in whom a permanent pacemaker is indicated.