The purpose of this study was to record median frequency of ventricular fibrillation (VF) in patients undergoing cardiopulmonary bypass for cardiac surgery, and to assess whether defibrillation success depends upon median VF frequency. Data were collected from 20 patients undergoing aortocoronary bypass grafting. Using computerized fast Fourier transformation of the signal from the electrogram, median VF frequency was assessed from onset of VF until aortic cross-clamping and during the 4-s period immediately before each defibrillation during the reperfusion phase. During VF, when an adequate coronary perfusion was maintained by cardiopulmonary bypass prior to aortic cross-clamping, median VF frequency (5.8 ± 0.1 Hz to 6.2 ± 0.1 Hz) remained constant for the entire observation interval (96 ± 25 s; mean ± SEM). A total of 42 defibrillations were performed: 22 resulted in supraventricular rhythm, 10 in VF, 6 in asystole, and 4 in electromechanical dissociation (EMD). Median VF frequency before defibrillation resulting in supraventricular rhythm was 4.7 ± 0.17 Hz. In contrast, median VF frequencies before unsuccessful defibrillation resulting in persistent VF (3.5 ± 0.28 Hz; P < 0.05), EMD (2.9 ± 0.15 Hz; P < 0.01), or asystole (2.8 ± 0.28 Hz; P < 0.01) were significantly lower. Above a threshold of 3.0 Hz, the probability of successful defibrillation increased as median VF frequency increased. The probability of success was 100% at a frequency of ≥5.5 Hz. We conclude that median VF frequency is a reliable noninvasive variable which can be used to predict defibrillation success during the reperfusion phase after cardiac surgery.