Endocardial catheter mapping of the right atrium during sinus rhythm and programmed atrial stimulation were performed in 50 patients with sick sinus syndrome to investigate the relationship between abnormal atrial electrograms recorded during sinus rhythm and some determinants of the atrial vulnerability such as repetitive atrial firing and fragmented atrial activity elicited by single extrastimulus. The patients were divided into 2 groups on the basis of the presence (Group I) or absence (Group II) of abnormal atrial electrograms recorded during sinus rhythm. In Group I (N= 32), repetitive atrial firing was induced in 23 (72%) patients, and in Group II (N= 18) in 6 (33%) patients; p less than 0.01. The repetitive atrial firing zone was 41 ± 37 ms in Group I and 12 ± 18 ms in Group II; p less than 0.001. Fragmented atrial activity was induced in 30 (94%) patients from Group I, and in 8 (44%) patients from Group II; p less than 0.0001. The fragmented atrial activity zone was 47 ± 42 ms in Group I and 14 ± 19 ms in Group II; p less than 0.0001. The atrial electrogram width at the premature beat (A2; p < 0.02) and the maximum A2/A1 ratio (p < 0.002) were 178 ± 53 ms and 196% ±40%, respectively in Group I, and 141 ±36 ms and 159% ± 30%, respectively in Group II. Atrial fibrillation was induced in 13 (41%) patients from Group I, and in 1 (6%) patient from Group II (p < 0.01). These data suggest a significantly greater vulnerability of the atrial muscle to atrial fibrillation in patients with sick sinus syndrome who possess abnormal atrial electrograms than in those who do not.