© 2018 American College of Emergency Physicians Instantaneous rigor is the immediate appearance of rigor mortis after cardiac arrest. To our knowledge, no previous reports exist on resuscitation of such patients. A young athlete suddenly collapsed with cardiac arrest during a marathon; his legs stiffened with instantaneous rigorlike stiffness. This stiffening provoked hyperkalemia, rhabdomyolysis, and multiple organ failure. We decided to amputate both legs, with venoarterial extracorporeal membrane oxygenation support. The patient recovered and was discharged without neurologic impairment. This rare case highlights the potentially significant effect of instantaneous rigor.