Introduction: Ventricular defibrillation is probabilistic and shock strength dependent. We investigated the relationship between defibrillation outcome and postshock activation patterns for shocks of the same strength (~50% probability of success for defibrillation [ED50] to yield an equal number of successful and failed shocks). Methods and Results: In five pigs, 10 shocks of ~ED50 strength (right ventricle-superior vena cava, biphasic, 6/4 msec) were delivered after 10 seconds of ventricular fibrillation (VF). Epicardial activation sequences following shocks were mapped with a 504-electrode sock and analyzed by animating dV/dt of the electrograms. Intercycle interval (ICI, time between the onset of successive postshock cycles), wavefront conduction time (WCT, time between the earliest and latest activation of a cycle), and overlapping index (WCT of cycle[n]/ICI of cycle[n+1]) were determined for the first five postshock cycles. An overlapping index > 1 indicates overlap between successive cycles. Of 50 defibrillation attempts, 25 were successes. There was no difference between successful and failed episodes for both ICI (68 ± 9 msec vs 62 ± 10 msec) and WCT (97 ± 24 msec vs 100 ± 14 msec) of cycle 1. However, starting at cycle 2, the ICI was longer, and the WCT was shorter for successful titan failed episodes (P < 0.01). Overlapping cycles. (index > 1) were found during the transition from cycles 2 through 5 in all failed (index >1) and in no successful episodes. Conclusions: (1) Defibrillation outcome cannot be determined during the first postshock cycle. (2) At least three rapid successive cycles with overlap of cycles 2 and 3 are present in all failed and in no successful episodes. (3) The overlapping index is a marker to predict defibrillation outcome.