<jats:p> <jats:bold> <jats:italic>Background—</jats:italic> </jats:bold> Surgical pulmonary endarterectomy is the preferred treatment for chronic thromboembolic pulmonary hypertension. Persistent pulmonary hypertension after pulmonary endarterectomy has been recognized as a major determinant of poor outcome. We tested whether acute vasoreactivity identifies chronic thromboembolic pulmonary hypertension patients prone to develop persistent/recurrent pulmonary hypertension after pulmonary endarterectomy and whether the degree of acute vasoreactivity affects survival or freedom from lung transplantation. </jats:p> <jats:p> <jats:bold> <jats:italic>Methods and Results—</jats:italic> </jats:bold> Right-sided heart catheterization at baseline and after inhalation of 40 ppm nitric oxide for 20 minutes was performed in 103 patients (56.3±15.3 years old, 53 women). Reductions in mean pulmonary arterial pressure (ΔmPAP; −8.8±12.6%; <jats:italic>P</jats:italic> <0.0001) and pulmonary vascular resistance (−16.1±18.1%; <jats:italic>P</jats:italic> <0.0001) and an increase in mixed venous saturation during inhaled nitric oxide (9.1±11.6%; <jats:italic>P</jats:italic> <0.0001) were observed. Sixty-two patients underwent pulmonary endarterectomy after a median of 49 days (25th and 75th percentiles: 24 and 123 days). Operated patients were followed up for a median of 70.9 months (25th and 75th percentiles: 14 and 97 months). Change in mPAP during inhaled NO was identified as a predictor of persistent/recurrent pulmonary hypertension after pulmonary endarterectomy. Patients experiencing a reduction in mPAP >10.4% with nitric oxide inhalation had a better postoperative outcome. A significant correlation was found between ΔmPAP and immediate postoperative pulmonary vascular resistance ( <jats:italic>r</jats:italic> =0.5, <jats:italic>P</jats:italic> <0.0001). </jats:p> <jats:p> <jats:bold> <jats:italic>Conclusions—</jats:italic> </jats:bold> A total of 80 (77.7%) of 103 patients demonstrated acute pulmonary vascular reactivity of some degree. A decrease in mPAP >10.4% under inhaled nitric oxide is a predictor of long-term survival and freedom from lung transplantation in adult patients with chronic thromboembolic pulmonary hypertension who are undergoing pulmonary endarterectomy. </jats:p>