Defects in the cardiac repolarization are associated to cardiac electrical instability but no repolarization index has proven sufficient sensitivity and specificity to justify preventive interventions yet. Aim of the present study was to propose a new repolarization frequency-based index, termed f99, and to test its predictive power on exercise 15-lead (X, Y, Z, V1 to V6, I to III, aVr, aVl, aVf) ECG recordings from 266 ICD patients, of which 76 developed ventricular tachycardia or fibrillation during the 4-year follow-up (ICD_Cases), and 190 did not (ICD_Controls). After performing the frequency-content evaluation of the repolarization signal (RPS) at the maximum heart-rate reached during exercise, f99 was computed as the frequency at which the RPS energy first reaches or overcomes 99%. Eventually, f99 single-lead values were maximized over lead-systems (MaxXYZ, MaxV1-V6 and Max12L), and predictive power of these parameters was evaluated as the area-under the receiver operating curve (AUC). Compared to the ICD_Controls, the ICD_Cases showed significantly higher values of MaxXYZ (38 vs. 27 Hz; AUC=0.61), MaxV1-V6 (42 vs. 33 Hz; AUC=0.60) and Max12L (46 vs. 37 Hz; AUC=0.65). Thus, the new f99 represents a promising tool to identify the risk of ventricular tachycardia or fibrillation, with Max12L showing the best predictive power.