Studies have shown that magnetic resonance imaging (MRI) does not have clinically important effects on the device parameters of non-MRI-conditional implantable cardioverter-defibrillators (ICDs). However, data on non-MRI-conditional ICD detection and treatment of arrhythmias after MRI are limited. To examine if non-MRI-conditional ICDs have preserved shock function of arrhythmias after MRI. Prospective cohort study. (ClinicalTrials.gov: NCT01130896). 1 center in the United States. 629 patients with non-MRI-conditional ICDs enrolled consecutively between February 2003 and January 2015. 813 total MRI examinations at a magnetic field strength of 1.5 Tesla using a prespecified safety protocol. Implantable cardioverter-defibrillator interrogations were collected after MRI. Clinical outcomes included arrhythmia detection and treatment, generator or lead exchanges, adverse events, and death. During a median follow-up of 2.2 years from MRI to latest available ICD interrogation before generator or lead exchange in 536 patients, 4177 arrhythmia episodes were detected, and 97 patients received ICD shocks. Sixty-one patients (10% of total) had 130 spontaneous ventricular tachycardia or fibrillation events terminated by ICD shocks. A total of 210 patients (33% of total) are known to have died (median, 1.7 years from MRI to death); 3 had cardiac arrhythmia deaths where shocks were indicated without direct evidence of device dysfunction. Data were acquired at a single center and may not be generalizable to other clinical settings and MRI facilities. Implantable cardioverter-defibrillator interrogations were not available for a subset of patients; adjudication of cause of death relied solely on death certificate data in a subset. Non-MRI-conditional ICDs appropriately treated detected tachyarrhythmias after MRI. No serious adverse effects on device function were reported after MRI. Johns Hopkins University and National Institutes of Health.