This study compared P-wave duration and dispersion (PD) in patients with short-term (less than or equal to48 hours) and long-term (>48 hours) atrial fibrillation (AF) after cardioversion. We studied 96 consecutive patients with short-term (group A; n:51, age: 61 +/- 11 years) and long-term AF (group B; n:45, age: 64 10 years). P-wave measurements were calculated from 12-lead electrocardiogram. There was no difference related to clinical characteristics. Left atrial dimension was significantly higher in group B (P = .003). P maximum (124 +/- 12 vs. 113 +/- 11 ms, P = .001) and PD (57 +/- 8 vs. 49 +/- 9 ms, P =.001) were also significantly longer in group B compared with group A, but P minimum did not. In univariate analysis, PD were related to AF duration (P = .002) and left atrial size (P = .02). This relation remained in multivariate analysis (P = .01, P = .02, respectively). P maximum >112 ms and PD >47 ms had accuracy values of 74% and 83% respectively for separating group B. Our results suggest that P wave duration and dispersion is prolonged in patients with long-term AF compared to short-term AF.