In patients with sick sinus syndrome, single chamber atrial pacing has been reported in retrospective studies to be associated with lower frequencies of atrial fibrillation, thromboembolism, heart failure, and mortality than ventricular pacing. We did a prospective randomised trial in 225 consecutive patients (142 women, 83 men; mean age 76 years) with the sick sinus syndrome, randomised to atrial (n = 110) or ventricular (n = 115) pacing and followed for up to five years (mean 40 [SD 18] months). During follow-up, the frequency of atrial fibrillation was higher in the ventricular group. Thromboembolic events (stroke or peripheral arterial embolus) occurred in 20 patients in the ventricular group and in six patients in the atrial group (p = 0.008). Twenty five patients died in the ventricular group compared with 21 in the atrial group (p = 0.74). The number of cases of heart failure did not differ between the two groups. Atrioventricular block occurred in two patients in the atrial group. It is concluded that patients with sick sinus syndrome should be treated with atrial pacing rather than ventricular pacing because atrial pacing is associated with lower frequencies of atrial fibrillation, thromboembolic complications, and a low risk of atrioventricular block.