Pulmonary hypertension is a relatively common complication of chronic lung disease in children that can cause diminished right ventricular performance (RVP) and, eventually, cor pulmonale and heart failure. Since oxygen may decrease pulmonary artery pressure in these patients, we questioned whether RVP would also improve concomitantly. We evaluated the effect of oxygen on RVP in two young hypoxemic patients by radionuclide angiography. A child with bronchopulmonary dysplasia and cor pulmonale who was not clinically in heart failure had acutely better RVP while breathing oxygen and a further improvement after continuous oxygen therapy for 1 year. In a young adult with cystic fibrosis who was suspected of being in heart failure RVP acutely improved when the FIO2 was increased. We conclude that oxygen may improve RVP in hypoxemic patients and speculate that the observation of such improvement may be valuable for the early detection of patients who can benefit from long-term oxygen therapy.