Aim of this study was to investigate the spatial patterns of organization of fibrillation in the right atrium using bipolar recordings from a basket catheter. Study population consists of 17 patients with persistent atrial fibrillation (AF), selected a priori for low energy internal cardioversion. Organization was measured by the number of occurrences--i.e. the percentage number of points laying on the baseline--which has been demonstrated to match Wells' criteria for the classification of AF. Well-defined and stable patterns of organization characterize the electrical activity of the right atrium in these patients. The organization in the entire right atrium showed an individual rather than common distribution. This may have implications in the choice of regions candidate for ablation.