Eleven of 21 patients having received more than 10 units of whole blood developed hyperpotassemia during the rapid phase of transfusion. The increase in serum potassium from initial values correlated well (r = 0.74) with the rate of the transfusion. Three of the hyperpotassemic patients developed cardiac arrest but no life-threatening arrhythmias were registered in the normokalemic patients. In contrast to most previous studies, transient hyperpotassemia often necessitating aggressive therapy was quite common in our massively transfused patients.