Objectives: The objective of this study was to evaluate the spatio-temporal organization and progression of human ventricular fibrillation (VF) in the left (LV) and right (RV) ventricles., Background: Studies suggest that localized sources contribute to VF maintenance, but the evolution of VF episodes has not been quantified., Methods: Synchrony between electrograms recorded from 25 patients with induced VF is computed and used to define the Asynchronous Index (ASI), indicating regions which are out-of-step with surrounding tissue. Computer simulations show that ASI can identify the location of VF-maintaining sources, where larger values of ASI max correlate with more stable sources., Results: Automated synchrony analysis shows elevated values of ASI in a majority of self-terminating episodes (LV: 8/9, RV: 7/8) and sustained episodes (LV: 11/11, RV: 12/12). The locations of ASI max in sustained episodes co-localize with rotor cores when rotational activity is simultaneously present in phase maps (LV: 8/8, RV: 5/7, p<.05). The distribution of ASI max differentiates self-terminating from sustained episodes (mean ASI max = 0.60±0.14 and 0.70±0.16, respectively; p=0.01). Across sustained episodes the LV exhibits an increase in ASI max with time., Conclusions: Quantitative analysis identifies localized asynchronous regions that correlate with sources in VF, with sustained episodes evolving to exhibit more stable activation in the LV. This successive increase in stability indicates a stabilizing agent may be responsible for perpetuating fibrillation in a "migrate-and-capture" mechanism in the LV.