Hypertrophic cardiomyopathy (HCM) is believed to be a leading cause of sudden cardiac death (SCD) in younger persons. Estimates of the incidence of SCD in patients with HCM vary widely, usually cited as 0.5-1%/year. Few prior studies have included autopsy and systematically sought details on preceding circumstances of death and prior history. Using the comprehensive Chief Coroner of Ontario database, encompasing all deaths attended by the coroner in Ontario, we identified all cases of SCD determined to be caused by HCM in individuals aged 10-65, between 2011-2016. EMS records, in-hospital information, death certificates, available medical records, coroner’s investigation statement, toxicology and post-mortem reports were reviewed. We identified 65 SCDs considered be caused by HCM over a 6-year period. Individuals older than 45 years comprised the vast majority of cases (64.6%). An autopsy was performed in all cases except for 8 cases with clinically diagnosed HCM. Based on a population of 10,125,401 Ontarians aged 10-65 (60,752,406 person years) and an estimated prevalence of HCM of 1:500, this corresponds to an estimated annual sudden death rate from HCM of 0.53/1,000 person years, and 0.37/1,000 person years amongst individuals ≤45 years (based on a population of 6,245,643 ; 37,473,858 person years). Males comprised 74.6% of the cases. Twenty-one individuals (31.8%) had a prior diagnosis of HCM. Eighty percent (53) of SCD events occurred at rest, 10.6% (6) during light activity (including walking and driving), 7.6%(5) during moderate exercise (including swimming, biking and fitness activities) and only 1.5% (1) during competitive sport. In 2 cases the circumstances could not be determined. Information about prior symptoms was available in 39 cases; 22 experienced symptoms during the week prior to SCD. In a validation data set, we reviewed all 911 attended cardiac arrests in the Greater Toronto Area (population of 4,128,570; 16,514,280 person years), ages 10-45, over a 4-year period and matched the EMS records with coroners files (89.1% matched) and autopsy information (82% matched). We identified 9 SCD from HCM, corresponding to an estimated incidence of 0.27/1000 person-years. The estimated incidence rate of SCD caused by HCM in ages 10-65 in Ontario is considerably lower than previously reported and is estimated at 0.05% per year amongst persons with HCM. The vast majority of events occurred during rest or light activity. Further studies are needed to assess the incidence and circumstances of SCDs in HCM.