Although electrophysiological abnormalities of atrial muscle have been evaluated in patients with paroxysmal atrial fibrillation (PAF), no prior study has determined the contribution of the patient's history of PAF to electrophysiological abnormalities. The study population consisted of 108 patients (71 men; mean age, 57 +/- 14 years) with symptomatic and idiopathic PAF who underwent electrophysiological study, Before electrophysiologicol study, histories of frequency, number of PAF episodes per month, and duration, a time interval from the first episode of PAF to electrophysiological study, were examined. At electrophysiological study, endocardiol electrograms from 12 right atrial sites were recorded during sinus rhythm, and the right atrial effective refractory period was determined. Longest duration of atrial electrograms, maximal number of fragmented deflections, and number of abnormal atrial electrograms recorded at the right atrial sites were significantly greater in the frequent group (> 1 PAF episode per month, n = 57) than in the infrequent group (< 1 PAF episode per month, n 51) (98 +/- 18 ms vs 88 +/- 16 ms, P < 0.005; 8.7 +/- 2.6 vs 7.5 +/- 2.6, P < 0.05; and 2.2 +/- 2.2 vs 1.4 +/- 1.6, P < 0.05, respectively). Indices of atrial vulnerability were also greater in the frequent group. Duration of PAF history was significantly correlated with longest duration r = 0.52, P < 0.0001), maximal number of fragmented deflections r = 0.51, P < 0.0001), and number of abnormal atrial electrograms r = 0.58, P < 0.0001). More frequent episodes and longer history of PAF significantly increased the electrophysiological abnormalities of the atrial muscle, suggesting that PAF results in gradual electrical remodeling of the atrial muscle.