This prospective study assessed factors affecting changes in potassium levels among 125 critically ill patients who received 160 units of packed red blood cell transfusions. The primary outcome was development of hyperkalemia, defined as a potassium level of 5.5 mEq/L or higher, after transfusion. The mean potassium increased slightly from 3.9 to 4.3 mEq/L post-transfusion but this change was not statistically significant. Four percent of patients developed hyperkalemia. Univariate and multivariate analysis found that transfusion of older stored blood (>12 days) was the only statistically significant factor associated with increased potassium levels. No other factors such as acidosis, renal failure, or medications significantly affected potassium changes.