Because many episodes of ventricular fibrillation (VF) are believed to be triggered by ventricular tachycardia (VT), patients who present with VT or VF are usually grouped together in discussions of natural history and treatment. However, there are significant differences in the clinical profiles of these 2 patient groups, and some studies have suggested differences in their response to therapy. We examined arrhythmias occurring spontaneously in 449 patients assigned to implantable cardioverter-defibrillator (ICD) therapy in the Antiarrhythmics Versus Implantable Defibrillators (AVID) trial to determine whether patients who receive an ICD after VT have arrhythmias during follow-up that are different from patients who present with VF. ICD printouts were analyzed both by a committee blinded to the patients’ original presenting arrhythmia and by the local investigator. During 31 ± 14 months of follow-up, 2,673 therapies were reported. Patients who were enrolled in the AVID trial after an episode of VT were more likely to have an episode of VT (73.5% vs 30.1%, p <0.001), and were less likely to have an episode of VF (18.3% vs 28.0%, p = 0.013) than patients enrolled after an episode of VF. Adjustment for differences in ejection fraction, previous infarction, and β-blocker and antiarrhythmic therapy did not appreciably change the results. Ventricular arrhythmia recurrence during follow-up is different in patients who originally present with VT than in those who originally present with VF. These findings suggest there are important differences in the electrophysiologic characteristics of these 2 patient populations.