Hypokalemia occurs in 50% to 68% of trauma patients. To investigate the pathophysiology of these changes in serum potassium, we prospectively studied 133 trauma patients. Among the patients who had hypokalemia, the serum potassium usually decreased within 1 hour of trauma and returned to normal within 24 hours without significant potassium replacement. In bivariant analysis, age, admission systolic blood pressure, cardiac injury, and serum epinephrine level were associated with admission serum potassium value (Kl), whereas sex, mechanism of injury, number of organ systems injured, blood glucose, serum alcohol, arterial pH, Injury Severity Score, trauma score, estimated blood loss, and urine potassium were not significantly related to Kl. But in a multiple regression model, the only significant independent variables were age, arterial pH, and serum epinephrine level.