The cardiac catheterization data of six infants with bronchopulmonary dysplasia (BPD) were reviewed to examine the responsiveness of their pulmonary vascular beds to changes in oxygen tension. The infants were studied because of slow recovery from their oxygen requirements and clinical evidence of persistent pulmonary hypertension. All were receiving home oxygen therapy and had abnormal chest radiographs and right ventricular hypertrophy by ECG at the time of catheterization (mean age, 25 months). All infants had mean pulmonary artery pressure greater than 25 mm Hg in room air, with a mean of 48 mm Hg. All decreased mean pulmonary artery pressure by at least 10 mm Hg when placed in high levels of inspired oxygen (FiO2 greater than 80), with a mean pulmonary artery pressure of 25 mm Hg. This represented a significant decrease in mean pulmonary artery pressure from room air pressures (P less than .005). Mean pulmonary artery pressure was also measured in three infants who were breathing supplemental oxygen by nasal cannula at flow rates similar to levels used for outpatient therapy. Most of the reduction in mean pulmonary artery pressure that occurred at high FiO2 occurred at these lower flow rates of supplemental oxygen. It is concluded that infants with bronchopulmonary dysplasia who have pulmonary hypertension generally have reactive pulmonary vascular beds, responsive to supplemental oxygen. Continuous oxygen therapy by nasal cannula may be useful in the treatment of pulmonary hypertension associated with bronchopulmonary dysplasia.