All 3825 out-of-hospital cardiac arrests that occurred among persons 12 to 45 years of age from 2009 through 2014 and were reported in the Rescu Epistry database were filtered on the basis of the type of location (e.g., a recreational facility) of the cardiac arrest. All 2144 remaining cases were then manually filtered for out-of-hospital cardiac arrests that occurred during or within 1 hour after a sports activity. This remaining cohort of 74 cases was assessed comprehensively with the use of all available records to adjudicate the cause of sudden cardiac arrest; cases of cardiac arrest due to noncardiac causes (e.g., hanging, trauma, mechanical suffocation, asphyxia from drowning, or toxic inhalation) were excluded from the assessment. EMS denotes emergency medical services.