Background: The diagnostic work-up for cardiac arrest from ventricular tachyarrhythmias occurring in younger adults and structurally normal hearts is variable and often incomplete. Methods: We reviewed records for all recipients of a secondary prevention implantable cardiac defibrillator (ICD) younger than 60 years at a single quaternary referral hospital from 2010-2021. Patients were included if they had unexplained ventricular arrhythmias (UVA) and absence of structural heart disease on echocardiogram, normal coronary assessment and no clear diagnostic features on ECG. We specifically evaluated the adoption rate of five modalities of ‘second-line’ cardiac investigations: cardiac magnetic resonance imaging (CMR), exercise ECG, flecainide challenge, electrophysiology study (EPS) and genetic testing. We also evaluated patterns of anti-arrhythmic drug therapy and device-detected arrhythmias and compared them with secondary prevention ICD recipients with a clear aetiology found on initial assessment. Results: 102 recipients of a secondary prevention ICD under the age of 60 were analysed. 39 patients (38.2%) were identified with UVA and were compared with the remaining 63 patients with VA of clear aetiology (61.8%). UVA patients were younger (35.6 ± 13.0 years vs 46.0 ± 8.6 years, p<0.001) and were more often female (48.7% vs 28.6%, p=0.04). CMR was performed in 32 patients with UVA (82.1%), whereas flecainide challenge, stress ECG, genetic testing and EPS were only performed in a minority of patients. Overall, the use of a second-line investigation suggested an aetiology in 17 patients with UVA (43.5%). Compared to patients with VA of clear aetiology, UVA patients had lower rates of antiarrhythmic drug prescription (64.1% vs 88.9%, p=0.003) and had a higher rate of device-delivered tachy-therapies (30.8% vs 14.3%, p=0.045). Conclusion: In this real-world analysis of patients with UVA, the diagnostic work-up is often incomplete. While CMR was increasingly utilized at our institution, investigations for channelopathies and genetic causes appear to be underutilized. Implementation of a systematic protocol for work-up of these patients requires further study.