Inappropriate shock is a frequently seen clinical problem despite advanced technologies used in modern implantable cardioverter-defibrillator (ICD) devices. Our aim was to investigate whether simply raising the ICD detection zones can decrease inappropriate therapies while still providing appropriate therapy. We randomized 223 patients with primary prevention to either the conventional programming group with 3 zones as VT₁ (167 to 182 beats/min) with discriminators, VT₂ (182 to 200 beats/min) with discriminators, and ventricular fibrillation (>200 beats/min) (n = 100) or the high-zone programming group with 3 zones as VT₁ (171 to 200 beats/min) with discriminators, VT₂ (200 to 230 beats/min) with discriminators, and ventricular fibrillation (>230 beats/min; n = 101). Twenty-two patients were lost to follow-up. The primary objectives were the first episode of appropriate and inappropriate therapies. The secondary objectives were all-cause mortality and hospitalization for heart failure. During 12-month follow-up, the first episode of appropriate therapy was higher (22% vs 10%, hazard ratio [HR] 2.18, 95% confidence interval [CI], 1.09 to 4.36, p = 0.028) and the first episode of inappropriate therapy was lower (5% vs 28%, HR 0.18 [95% CI 0.07 to 0.44], p <0.001) in the high-zone group compared with the conventional group. Although all-cause mortality did not differ (2% for the high-zone group vs 3% for the conventional group, HR 0.65 [95% CI 0.11 to 3.99], p >0.05), hospitalization for heart failure was significantly higher in the conventional group (13% vs 4%, HR 0.28 [95% CI 0.09 to 0.88], p = 0.021). In conclusion, in a real-world population, high-zone settings of the single-, dual-, and triple-chamber ICDs were associated with reduction in inappropriate therapy while still providing appropriate therapy.