The relationship between the degree of electrical organization of ventricular fibrillation (VF) and defibrillation success was investigated in this study using a new technique to quantify organization--spatial coherence. This technique employs the use of the magnitude-squared coherence spectrum to analyze multichannel electrograms obtained during a cardiac mapping study. Magnitude-squared coherence values for all possible pairs of electrograms recorded from an epicardial plaque consisting of 112 electrodes were computed. Average coherence was plotted versus electrode separation distance, and the data were fit with an exponentially decaying curve. Two parameters indicative of myocardial organization were extracted from the curve. The coherence length (d) was defined as the distance (mm) at which the average coherence dropped to a given level, and the coherence strength was defined as the average coherence value at a given distance. Higher values for these parameters were hypothesized to indicate higher levels of organization. The spatial coherence technique was tested previously in a canine study of ventricular fibrillation (VF) and normal sinus rhythm, and the results suggested that spatial coherence parameters may be used to compare cardiac rhythms in terms of their organization. To test the hypothesis that organization is related to defibrillation success, 164 mapping sessions recorded during repeated VF induction and defibrillation trials using a monophasic waveform were performed in a close-chested canine study (n = 9) using a fixed energy and VF duration (10 seconds). Three coherence lengths and five coherence strengths were calculated for each VF episode. Results using a two-way analysis of variance with blocking between dogs showed that all of the coherence length and three of the coherence strength parameters were higher for those VF episodes that were successfully defibrillated than for those that were not (P < .05). Energy delivered and transmyocardial impedance were not significantly different between the groups. The authors conclude (1) the organization of a VF episode, as reflected in the spatial coherence parameters, is related to defibrillation success and may be partially responsible for the probabilistic nature of defibrillation and (2) the spatial coherence technique provides a means of quantifying myocardial electrical organization and is an important experimental tool that may be used to obtain a better understanding of VF and its termination.