The detection of paroxysmal atrial fibrillation (AF) in patients requiring a permanent pacemaker for sinus node disease may influence the choice of both pacemaker and programmed mode. While signal-averaged ECG of the P wave (SAEP) during sinus rhythm may detect patients with paroxysmal AF, the value of SAEP in the presence of sinus node disease is unknown. We therefore evaluated SAEP in patients with sinus node disease during sinus rhythm and atrial pacing. We investigated 10 patients with sinus node disease alone (SND), 10 with sinus node disease and paroxysmal AF (SND-PAF), and 20 normal controls (NC) using a P wave specific, signal-averaging system. In sinus rhythm, duration and energy were greater in SND-PAF than in SND and NC (mean [SEM] duration: 153 [4] msec, 140 [4] msec, and 134 [2] msec, P < 0.001; energy from 20-150 Hz: 76 [18] muV2.sec, 48 [7] muV2.sec, and 36 [3] muV2.sec, P = 0.006). Atrial pacing in SND-PAF produced an 11% prolongation of atrial activation but little further abnormality in P wave characteristics. In SND, atrial pacing caused a 20% prolongation of the P wave and increased P wave energy to a greater extent than in SND-PAF. We conclude that in patients with SND, atrial activation appears similar to normal controls during sinus rhythm but changes significantly on pacing. In patients with SND-PAF, atrial activation is abnormal during sinus rhythm with little further change when the atrium is paced. SAEP may be useful in detecting a predisposition to paroxysmal AF in the presence of sinus node disease and might help optimize pacemaker prescription.