The authors investigated the safety of oral tetrahydrobiopterin (BH4), a cofactor for nitric oxide synthesis, as a novel treatment for pulmonary hypertension (PH). Eighteen patients with pulmonary arterial hypertension or inoperable chronic thromboembolic PH received sapropterin dihydrochloride (6R-BH4), the optically active form of BH4, in addition to treatment with sildenafil and/or endothelin receptor antagonists in an open-label, dose-escalation study. 6R-BH4 was administered starting at a dose of 2.5 mg/kg and increasing to 20 mg/kg over 8 weeks. Changes in markers of nitric oxide synthesis, inflammation and oxidant stress, as well as exercise capacity and cardiac function were measured. 6R-BH4 was well tolerated at all doses without systemic hypotension, even when given in combination with sildenafil. There was a small but significant reduction in plasma monocyte chemoattractant protein (MCP)-1 levels on 5 mg/kg. No significant changes in measures of nitric oxide synthesis or oxidant stress were observed. There was improvement in 6-minute walk distance, most significant at a dose of 5 mg/kg, from 379 ± 61 to 413 ± 57 m 414 ± 57 m (P = .002). Oral 6R-BH4 can be administered safely in doses up to 20 mg/kg daily to patients with PH. Further studies are needed to explore its therapeutic potential.