Background: Despite clinical benefits of implantable cardioverter-defibrillator (ICD), inappropriate defibrillator shocks are common adverse consequences that may cause psychological distress. However, limited data are available on inappropriate shock in Korea. We investigated the incidence and clinical outcome of inappropriate shock in patients who had received an ICD. Methods: Using a retrospective, single-center registry between October 1999 and January 2011, 122 patients were selected. An inappropriate shock was defined as an episode during which 1 or more inappropriate shocks occurred. Primary outcome was a composite event of all-cause mortality or cardiac-cause hospitalization. Results: Median follow-up duration was 33 (interquartile range: 14 to 52) months. One or more inappropriate shocks occurred in 29 (23.8%) patients. Appropriate shock occurred in 4 (19.0%) and 56 (55.4%) of patients who had received an ICD for primary and secondary prevention of sudden cardiac death, respectively (p=0.013). Inappropriate shock occurred in 5 (23.8%) for primary prevention and 24 (23.8%) for secondary prevention, respectively (p=0.692). A history of atrial fibrillation was an only independent predictor of the occurrence of inappropriate shock (hazard ratio [HR]: 2.96, 95% confidence interval [CI]: 1.10-7.99, p=0.032). Atrial fibrillation was the most common cause for inappropriate shock (69.0%), followed by supraventricular tachycardia (20.7%), and then erroneous sensing (10.3%). A composite event of all-cause mortality or cardiac-cause hospitalization during follow-up was not significantly different among patients with or without inappropriate shock (inappropriate shock vs. no inappropriate shock: 37.9% vs. 25.8%, adjusted HR 1.61, 95% CI 0.69-3.73, p=0.272). Conclusions: Inappropriate shocks do not affect clinical outcomes in patients who had received an ICD, although incidence of inappropriate shocks still remains high.