BACKGROUND Algorithms to reject irregular tachyarrhythmias are available in implantable cardioverter-defibrillator devices to discriminate ventricular tachycardia (VT) from atrial fibrillation (AF). The hazard of underdetection of irregular monomorphic VTs using these algorithms has not yet been fully evaluated. The purpose of this study was to determine the ability of a commonly used stability algorithm to reject AF and to correctly detect VT with a high RR interval variability. RESULTS The electrophysiological studies from 232 patients with induced monomorphic VT (cycle length > 250 ms) and 21 with AF were reviewed. A preliminary analysis was performed to classify the VT episodes in irregular (successive RR differences > 20 ms after 4 seconds from onset) or regular (otherwise). Three study groups were defined: group 1 (27 patients with irregular VT), group 2 (22 randomly selected patients with regular VT), and group 3 (21 patients with AF). A computer program analyzed the first 50 RR intervals of the induced VT (AF), resetting a VT counter if the interval was greater than a tachycardia detection interval (TDI) or if its absolute difference with the preceding three beats exceeded a programmed stability value (STAB). The VT was detected when the VT counter reached a preset number of intervals (NIDs). Different combinations of TDI, STAB, and NID were analyzed. All VTs in group 2 were correctly detected. In contrast, up to 10 VTs from group 1 were not detected when high NIDs and low STAB parameters were programmed. With usual values (10 to 16 beats and 50 to 60 ms, respectively), only 1 to 2 VTs remained undetected, but 20% to 50% had a detection delay > 8 seconds. Undetected VTs were significantly slower than early detected VTs for most STAB and NID combinations. With usual stability and NID values, 10% to 20% of episodes of AF were inappropriately detected. Changes in TDI had a small impact on sensitivity and specificity when currently used values for stability were programmed. CONCLUSIONS Animplantable cardioverter-defibrillator tachycardia detection algorithm with a stability criterion of 50 to 60 ms and 12 to 14 RR intervals is able to detect over 90% of monomorphic irregular VTs. Nevertheless, significant VT detection delays may arise, and inappropriate detection of AF cannot be totally prevented.