Bosentan, an endothelin-1 antagonist that can be administered orally, has been shown to be effective in the treatment of idiopathic pulmonary arterial hypertension and may be of benefit to patients with the Eisenmenger syndrome. Nine patients with Eisenmenger's syndrome were treated with bosentan at a dose of 125 mg twice a day. After treatment with bosentan, 6 of 9 patients (67%) had an improvement in New York Heart Assocation classification of ≥1 grades (p = 0.03). Oxygen saturation levels increased from 79 ± 5% to 88 ± 6%, (p = 0.03). The side effects of bosentan therapy were minor; no significant changes in liver function tests were noted. These preliminary data suggest that oral administration of bosentan therapy for Eisenmenger's syndrome results in improved oxygenation and functional status with minimal side effects.