A 60-year-old woman in severe hemorrhagic shock underwent urgent laparotomy to control massive hematemesis. Severe metabolic acidosis due to hemorrhagic shock and hyperkalemia as well as hypocalcemia associated with rapid blood transfusion were aggressively corrected with administration of sodium bicarbonate, insulin, and calcium chloride. Following rapid transfusion of the last 8 units of red cell concentrate (RCC), however, cardiac arrest occurred because of hyperkalemia and did not respond to cardiopulmonary resuscitation. Blood gas analysis revealed that the serum K⁺ concentration had increased from 4.05 to 8.24 mEq/L over a 7-minute period, while the Ca²⁺ concentration had decreased from 1.43 to 0.53 mmol/L. Rapid transfusion of irradiated RCC containing a high concentration of K⁺, an extreme decrease in the circulating blood volume to dilute the exogenously administered K⁺ and citrate, and severe metabolic acidosis impeding the intracellular shift of K⁺ seemed to have contributed to the extremely rapid development of fetal hyperkalemia accompanied by hypocalcemia. Anesthesiologists must be aware that hyperkalemia due to rapid blood transfusion can develop extremely rapidly in patients in severe hemorrhagic shock.