Sudden death from cardiac arrest in a young person may occur during sports play after a blunt blow to the chest in the absence of structural cardiovascular disease or traumatic injury (cardiac concussion or commotio cordis). We studied the clinical features of this apparently uncommon but important phenomenon. We identified cases from the registries of relevant agencies and organizations, as well as newsmedia accounts, and developed a clinical profile of 25 children and young adults, 3 to 19 years of age. Each victim collapsed with cardiac arrest immediately after an unexpected blow to the chest, which was usually inflicted by a projectile (such as a baseball or hockey puck). Incidents took place during organized competitive sports in 16 cases and in recreational settings at home, at school, or on the playground in 9. In each instance, the impact to the chest was not judged to be extraordinary for the sport involved and did not appear to have sufficient force to cause death. Twelve victims collapsed virtually instantaneously on impact, whereas 13 remained conscious and physically active for a brief time before cardiac arrest. Cardiopulmonary resuscitation was administered within about three minutes to 19 victims, but normal cardiac rhythm could be restored in only 2 (both incurred irreversible brain damage and died shortly thereafter). Seven victims (28 percent) were wearing some form of protective chest padding. We speculate that most sudden deaths related to impact to the chest (not associated with traumatic injury) are due to ventricular dysrhythmia induced by an abrupt, blunt precordial blow, presumably delivered at an electrically vulnerable phase of ventricular excitability. This profile of blunt chest impact leading to cardiac arrest adds to our understanding of the range of causes of sudden death on the athletic field and may help in the development of preventive measures.