Out of a pacemaker clinic population of 182 patients 21 (11.5%) were found to have the sick sinus syndrome. Their ages ranged from 30 to 80 years and averaged 62. Ischaemic heart disease was more commonly an aetiological factor than in patients with chronic atrioventricular heart block. Other aetiologies were familial cardiomyopathy, postcardiac surgery, and dystrophia myotonica.Cardioversion before pacemaker insertion was a hazardous procedure. After insertion the occurrence of tachycardias and the need for drug therapy were reduced. One patient no longer required a pacemaker once atrial fibrillation became established.A high incidence of cerebral embolization was observed and the use of anticoagulant drugs therefore merits serious consideration.Failure of inhibition of demand-type pacemakers occurred in two patients. Two patients who exhibited competition with fixed-rate pacemakers died. Two patients were treated with electrodes surgically implanted on the right atrium. It is suggested that fixed-rate pacemakers may be contra-indicated and that sequential atrioventricular demand pacing is theoretically ideal.