Abnormal atrial refractoriness was examined as a cause of atrial fibrillation/flutter (AFF) in patients with bradycardia. Refractory periods at three disparate right atrial sites were compared in 17 patients with sinus node dysfunction (SND) and 16 controls. Atrial pacing shortened refractory periods, but failed to decrease dispersion of refractoriness significantly. During sinus rhythm, duration and dispersion of refractoriness were greater in SND patients than in controls. These differences persisted with atrial pacing. For example, at the paced rate, dispersion of effective refractory periods in SND patients was greater than in controls (62.9 ± 34 vs 36.6 ± 21 msec,p < 0.01). Six SND patients had AFF, but they did not have greater dispersion than other SND patients, or unusually short or long refractory periods. Thus, prolonged and nonuniform refractoriness were features of SND. Abnormal refractoriness in SND reflected atrial disease and persisted with pacing. These abnormalities were not unique to patients with AFF.