Abstract Aims The aim of this study is to compare clinical characteristics and causes of death among witnessed and unwitnessed sudden cardiac death (SCD) cases aged 1–35 years. Methods and results In this retrospective nationwide study, all deaths in persons aged 1–35 years in Denmark during 2000–09 were included (23.7 million person-years). Using the in-depth descriptive Danish death certificates and Danish nationwide registries, 860 cases of sudden, unexpected death were identified. Through review of autopsy reports and register data, we identified 635 cases of SCD of which 266 (42%) were witnessed and 326 (51%) were unwitnessed. In 43 cases (7%), witnessed status was missing. Clinical characteristics were overall similar between the two groups. We found a male predominance among unwitnessed SCD compared to witnessed SCD (71% and 62%, respectively, P-value 0.012), as well as more psychiatric comorbidity (20% and 13%, respectively, P-value 0.029). Unwitnessed SCD more often occurred during sleep whereas witnessed SCD more often occurred while the individual was awake and relaxed (P-value < 0.001). The autopsy rate among all SCD cases was 70% with no significant difference in autopsy rate between the two groups. Sudden unexplained death, which was the leading autopsy conclusion in both groups, was more frequent among unwitnessed SCD (P-value 0.001). Conclusion Several clinical characteristics and autopsy findings were similar between witnessed and unwitnessed SCD cases. Our data support the inclusion of both witnessed and unwitnessed cases in epidemiological studies of SCD cases aged 1–35 years, although the risk of misclassification is higher among unwitnessed and non-autopsied cases of SCD.