The avoidance of inappropriate shock therapy is an important clinical issue in implantable cardioverter-defibrillator (ICD) patients. We retrospectively analyzed therapeutic events in ICD patients, and the effect of tachycardia detection interval (TDI) and tachycardia cycle length (TCL) guided reprograming on the reduction of inappropriate ICD therapy. The clinical determinants of after reprogramming were also evaluated.A total of 254 consecutive ICD patients were included in the study, and the incidence of antitachycardia therapy was evaluated during the follow-up period of 27.3 ± 18.7 months. When inappropriate antitachycardia therapy appeared, TDI was reprogrammed not to exceed the detected TCL and the patients continued to be followed-up. Various clinical parameters were compared between patients with and without inappropriate ICD therapy. During the initial follow-up period of 18.6 ± 15.6 months, ICD therapy occurred in 127/254 patients (50%) including inappropriate antitachycardia pacing (ATP) (12.9%) and shock (44.35%). Determinants of initial inappropriate therapy were dilated cardiomyopathy (DCM), history of therapeutic hypothermia, and QRS duration. Of the 61 patients with inappropriate therapy, 24 received TCL guided reprogramming. During the additional observation period of 17.0 ± 16.8 months, inappropriate therapy recurred in 5/24 patients (2 ATP, 3 shocks). The determinant of these inappropriate therapy events after reprogramming was the presence of supraventricular tachycardia.By applying simple TCL and TDI guided reprogramming, inappropriate therapy was reduced by 79%. The determinant of inappropriate therapy after reprogramming was the presence of supraventricular tachycardia.