A patient with severe interstitial pulmonary fibrosis, hypoxemia, pulmonary hypertension, and cor pulmonale was given inhaled nitric oxide (NO) followed by intravenous PGE1 to assess the reversibility of pulmonary hypertension. During NO inhalation, there was marked reduction in pulmonary vascular resistance, increased cardiac output, and dramatic improvement in arterial oxygenation. There was no effect on systemic vascular resistance. In contrast, intravenous PGE1 led to rapid arterial oxygen desaturation and worsened dyspnea. The beneficial responses to inhaled NO in this patient suggest that, even in severe chronic lung disease, reversible pulmonary vasoconstriction is present. Inhaled NO thus has a potential therapeutic role as a selective pulmonary vasodilator in patients with interstitial pulmonary fibrosis and cor pulmonale.