Background— Knowledge of the burden and causes of sudden cardiac death (SCD) is sparse in persons aged <50 years; better understanding is needed to lower the risk of SCD. The aim of this study was to report SCD incidence rates and autopsy findings in persons aged 1 to 49 years. Methods and Results— All deaths in persons aged 1 to 49 years were included in 2007 to 2009. Death certificates were reviewed by 2 physicians. History of previous admissions to hospital was assessed, and discharge summaries were read. Sudden unexpected death cases were identified and autopsy reports were collected. In the 3-year study period, there were 7849 deaths of which we identified 893 (11%) SCD cases. The annual incidence rate per 100 000 persons increased from 2.3 (95% confidence interval, 2.0–2.7) to 21.7 (95% confidence interval, 20.2–23.4) in persons aged 1 to 35 and 36 to 49 years, respectively. Coronary artery disease was the most common cause of death and was found in 158 (36%) autopsied cases, followed by 135 (31%) cases of sudden unexplained death. Conclusions— In a nationwide cohort of persons aged <50 years, the annual incidence rate of SCD was ≈10× higher in persons aged 36 to 49 years than in persons aged 1 to 35 years. Notably, coronary artery disease was the most common cause of SCD, followed by unexplained deaths. These findings may help in developing strategies to prevent SCD in the future.