Background Assessment of pulmonary vasodilator responsiveness is important in determining the prognosis and management of patients with pulmonary hypertension. Many patients, however, are already on vasodilators at the time of testing. It is unclear if these agents should be temporarily discontinued to improve the sensitivity of testing.Methods We examined the hemodynamic effects of nitric oxide (NO) inhalation in 60 patients with pulmonary arterial hypertension.