The relationship between abnormal atrial electrograms (AAE) recorded during sinus rhythm by endocardial catheter mapping of the right atrium and the atrial conduction defects of sinus impulses or single atrial extrastimuli was investigated in 44 patients with sick sinus syndrome. The patients were divided into two groups on the basis of the presence (n = 29) or absence (n = 15) of AAE recorded during sinus rhythm. The P wave duration in the AAE (+) Group patients was 137 +/- 14 msec, and 125 +/- 15 msec in the AAE (-) Group; P < 0.02. The intraatrial conduction time of sinus impulses in the AAE (+) Group was 54 +/- 12 msec, and 39 +/- 9 msec in the AAE (-) Group; P < 0.001. The interatrial conduction time in the AAE (+) Group was 101 +/- 14 msec, and 78 +/- 16 msec in the AAE (-) Group; P < 0.001. In the AAE (+) Group, 11 (38%) patients had a sinus node recovery time > 4 seconds, whereas in the AAE (-) Group there was only one (6%) patient; P < 0.03. AAE showed a specificity of 93% and a positive predictive accuracy of 91% in predicting inducibility of atrial fibrillation. The sensitivity was 35% and the negative predictive accuracy was 42%. Sustained atrial fibrillation was induced in ten (35%) patients of the AAE (+) Group, and in one (7%) patient of the AAE (-) Group; P < 0.05. These data suggest that in patients with sick sinus syndrome who possess abnormal endocardial electrograms in sinus rhythm within the right atrium have: (1) a significantly longer P wave duration; (2) a significantly longer intraatrial and interatrial conduction time of sinus impulses; and (3) a significantly greater sinus node dysfunction and higher incidence of induction of sustained atrial fibrillation. It is concluded that there are significantly greater atrial conduction defects in patients with sick sinus syndrome who possess AAE within the right atrium during sinus rhythm.