This study investigated the incidence of failed shock therapy in implantable cardioverter defibrillator (ICD) patients despite adequate defibrillation threshold (DFT) testing. The study analyzed data from 2738 ICD patients implanted between 2004-2013. 38 patients (1.4%) experienced at least one failed shock therapy, even though all 38 had adequate DFT testing with successful shocks over 10J below the programmed energy level. There was a trend towards a longer interval between DFT testing and failed shocks compared to patients experiencing only effective shocks. The results suggest that adequate DFT testing does not rule out failed shock therapy in some patients, supporting abandoning routine DFT testing in unselected patients.