Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmia syndrome characterized by adrenergically induced ventricular arrhythmias in patients with structurally normal hearts. Initiating triggered arrhythmias, such as bidirectional ventricular tachycardia , often degenerate into reentrant arrhythmias, such as ventricular fibrillation (VF). To determine whether the effectiveness of implantable cardioverter-defibrillator (ICD) shocks is dependent on rhythm type. It is a retrospective study of patients with CPVT who had undergone ICD implantation. Thirteen patients received ICDs (median age 15 years; range 9–43 years): 7 of 13 (54%) for cardiac arrest and 6 of 13 (46%) for syncope despite drug therapy . The median follow-up duration was 4.0 years (range 1.7–19.9 years). Nineteen reinterventions occurred, excluding generator replacements. Ten patients received 96 shocks (median 4 shocks; range 1–30 shocks). Eighty-seven shock electrograms were reviewed. Sixty-three (72%) shocks were appropriate, and 24 (28%) were inappropriate (T-wave oversensing 7 [29%], supraventricular arrhythmia 16 [67%], after self-terminating VF 1 [4%]). Among appropriate shocks, 20 (32%) were effective in terminating sustained arrhythmia and 43 (68%) were ineffective. Shocks delivered to triggered arrhythmias nearly always failed (1 of 40 [3%] effective), while shocks delivered to VF were usually successful (19 of 23 [83%] effective; P < .001). Four patients received 17 appropriate antitachycardia pacing therapies for ventricular tachycardia : only 2 (12%) were effective. No patient died. The effectiveness of ICD shock therapy in CPVT depends on the mechanism of the rhythm treated. Shocks delivered to initiating triggered arrhythmias nearly always fail, whereas those for subsequent VF are usually effective. ICD programming in these patients is exceptionally challenging.