To investigate the etiology of the hypokalemia that is more commonly associated with massive transfusions than hyperkalemia, we determined the plasma potassium concentrations of 26 stored packed cell preparations and reviewed a series of 15 patients who received massive transfusions with whole blood or packed cells. Our results confirm that stored packed cell preparations, like stored whole blood preparations, are hyperkalemic, and suggest that metabolic alkalosis, catecholamine release, and hemorrhagic shock are important factors in the development of hypokalemia associated with massive blood transfusions. In view of the association of hypokalemia and hyperkalemia with massive transfusion, plasma potassium levels should be carefully monitored in patients receiving massive transfusions.