The objective of this study was to determine whether a combination of inhaled nitric oxide (iNO) and O₂ is more effective than 100% O₂ or iNO alone for acute vasodilator testing in children. An open, prospective, randomized, controlled trial was conducted at 16 centers. Subjects were children 4 weeks to 18 years of age with pulmonary hypertension (PH) and increased pulmonary vascular resistance (PVR) undergoing right heart catheterization for acute vasodilator testing. All patients were tested with each of three agents (80 ppm iNO, 100% O₂, combination of 80 ppm iNO/100% O₂) in three 10-min treatment periods, and hemodynamic measurements obtained. Primary outcome measures were percentages of acute responders with O₂ alone vs. iNO/O₂ and iNO alone vs. iNO/O₂. More patients on the combination were acute responders compared with O₂ or iNO alone (26% vs. 14%, P = 0.019, and 27% vs. 24%, P = 0.602, respectively). Changes in PVR index and mean pulmonary arterial pressure vs. baseline were greater with iNO/O₂ vs. either O₂ or iNO alone (P < 0.001). Survival at 1-year follow-up included (1) 90.9% of acute responders to the combination, compared with 77.8% of nonresponders to the combination, and (2) 85.7% of acute responders to O₂ alone, compared with 80.6% of nonresponders to O₂. Key conclusions are as follows. In children with PH and increased PVR, more acute responders were identified with the iNO/O₂ combination vs. O₂ alone. While there was no significant difference in acute responder rate with iNO alone vs. iNO/O₂, the combination improved pulmonary hemodynamics acutely better than iNO alone. One-year survival data show similar rates between the iNO/O₂ and the O₂ alone groups; however, the combination may be more effective than O₂ alone in discriminating survivors versus nonsurvivors at long-term follow-up.