In patients with pulmonary disease, pulmonary artery hypertension often occurs as a result of pulmonary artery vasoconstriction, primarily from hypoxia and alveolar hypotension. In this report we describe the hemodynamic effects of breathing supplemental oxygen in a child with bronchopulmonary dysplasia and pulmonary artery hypertension. These hemodynamic effects include an improvement in oxygenation, an increase in systemic vascular resistance, and a decrease in the pulmonary vascular resistance. As a direct result of these changes in vascular resistances, alterations of heart rate, cardiac index, stroke volume, aortic pressure, oxygen consumption, and pulmonary artery pressure have been shown to occur. Oxygen is widely used to treat many physiologic conditions. However, during the administration of supplemental oxygen, rarely do we recognize the hemodynamic changes associated with its use. These hemodynamic effects must be clearly understood and appreciated before oxygen administration in any clinical situation.