A 3-month-old infant of 33 weeks' gestation was hospitalized with pneumonia caused by Bordetella pertussis. Respiratory insufficiency worsened, and on hospital day 3, there was severe pulmonary dysfunction (arterial oxygen pressure/fraction of inspired oxygen ratio: 120), extreme leukocytosis (white blood cell count 104,000/mm3), and severe pulmonary hypertension as assessed by 2-dimensional echocardiogram. A double volume exchange transfusion was performed to reduce the leukocyte mass. Oxygenation began to improve during the exchange and continued to improve over the ensuing 31 hours (arterial oxygen pressure/fraction of inspired oxygen ratio: 280). The white blood cell count fell dramatically after the exchange, and the rate of rise was slower after exchange therapy compared with preexchange.