Background-We investigated patterns of intramural activation in early human ventricular fibrillation (VF) and hypothesized that intramural reentry colocalizes to sites with increased intramural fibrosis. Methods and Results-Thirteen human Langendorff hearts were used for this study. Twenty-five plunge needles (4 unipoles/needle) were used to map 100 intramural sites. For the global mapping component, 11 20-s episodes of early VF were studied in 6 hearts. Simultaneous activation of all 4 electrodes was the most common pattern observed in 48.7% of needles, followed by an endocardial-to-epicardial activation pattern (9.8% of needles) and epicardial-to-endocardial activation pattern (5.5% of needles); 19.3% of needles had nonuniform multidirectional patterns. In 2 orthogonal planes, 1 parallel and 1 perpendicular to the epicardium and endocardium, reentry was detected in 14.3% of beats at any 1 level, and 5.8% of these were transmural. Simultaneous mapping of the epicardium and endocardium in 5 hearts detected concurrently rotating rotors with similar chirality and cycle length, suggesting the presence of transmural scroll waves (n=6), which was confirmed by high-resolution fixed-space mapping in 2 of those hearts plus 1 additional heart. Transmural optical mapping in 1 additional heart confirmed simultaneous epicardial and endocardial activation. Histopathology revealed greater fibrosis at sites of reentry compared to areas without (53.3±11.9% versus 27.5±2.4%, P=0.02). Conclusions-Intramural activation patterns suggest that early human VF does not organize as multiple reentrant wavefronts but is best explained by transmural scroll wave activation. Intramural reentry localizes to regions of greater intramural fibrosis. © 2011 American Heart Association, Inc.