VF Patterns in the Right and Left Ventricles. Introduction: The mechanisms that maintain ventricular fibrillation (VF) are not completely understood. It has been proposed that increased ventricular wall thickness destabilizes VF wavefronts and therefore is an important determinant of VF activation patterns. We hypothesized that if this is the case, then VF patterns on the thin-walled right ventricle (RV) should be simpler than those on the thick-walled left ventricle (LV). Methods and Results: in seven open chest pigs, we mapped VF simultaneously from two epicardial recording arrays, one on the RV and one on the LV. Each array contained 504 unipolar electrodes (in a 21 × 24 grid) spaced by 2 mm. We used specialized pattern analysis methods to compute quantitative descriptors of RV and LV activation patterns. Our data show that VF is more organized in the RV than the LV, containing fewer, larger wavefronts that follow fewer distinct pathways and are less likely to fragment or collide with other wavefronts. The incidence, size, and cycle length of reentrant circuits were similar in the two ventricles, but RV reentry persisted for more cycles. These results are not predicted by the differences in electrophysiologic properties between LV and RV that have been reported in mammalian hearts. Conclusion: The geometry of the ventricular wall, particularly wall thickness, is an important determinant of VF activation patterns.