To contextualize obesity rates in young sudden cardiac death (SCD) against the age-matched national population, and identify clinical and pathologic features in WHO class II and III obesity.A prospective state-wide out-of-hospital cardiac arrest registry included all SCDs in Victoria, Australia from 2019-2021. Body mass indices (BMIs) of patients 18-50 years were compared to age-referenced general population. Characteristics of SCD patients with WHO Class II obesity (BMI ≥30kg/m) and non-obesity (BMI<30kg/m) were compared. Clinical characteristics of people with BMI>50kg/m were assessed.2504 patients were included. Obesity was strongly over-represented in young SCD compared to the age-matched general population (55.0% vs 28.7%, p<0.0001). Obese SCD patients more frequently had hypertension, diabetes and obstructive sleep apnoea (p<0.0001, p=0.009 and p=0.001 respectively), ventricular fibrillation as their arrest rhythm (p=0.008) and left ventricular hypertrophy (LVH) (p<0.0001). Obese patients were less likely to have toxicology positive for illicit substances (22.0% vs 32.6%, p=0.008) or history of alcohol abuse (18.8% vs 26.9%, p=0.030). Patients with BMI>50 kg/m represented 8.5% of young SCD. LVH (n=26, 60.5%) was their predominant cause of death and only 10 (9.3%) patients died from coronary disease.2Over half of young Australian SCD patients are obese, with all obesity classes over-represented compared to the general population. Obese patients had more cardiac risk factors. Almost two thirds of patients with BMI>50 kg/m died from LVH, with fewer than 10% dying from coronary disease.2