Clinical studies have shown the importance of endothelin as a pathogenic mediator in pulmonary arterial hypertension (PAH). We describe the effects of bosentan, an oral dual endothelin receptor antagonist, in patients with PAH associated with human immunodeficiency virus (HIV) infection. In this prospective study, 16 patients with PAH associated with HIV infection in stable condition received bosentan for 16 weeks. Efficacy endpoints included exercise capacity, cardiopulmonary hemodynamics, Doppler echocardiography, New York Heart Association functional class, and quality of life (SF-36 and EQ-5D). Safety was assessed by laboratory tests, vital signs, and adverse events. Improvements were observed from baseline to Week 16 in all efficacy parameters: 6-minute walk distance (+91 +/- 60 m, p < 0.001), New York Heart Association class (14 patients improved), hemodynamics (cardiac index: +0.9 +/- 0.7 L/minute/m(2), p < 0.001), Doppler echocardiographic variables, and quality of life. During the study, no patient died and none required epoprostenol treatment. Hepatic tolerability was similar to that reported in patients with PAH. Bosentan had no negative impact on control of HIV infection. Although limited by uncontrolled design, small sample size and short duration, this study suggests that bosentan may benefit patients with PAH associated with HIV infection, and that endothelin is an important pathogenic mediator in this disease.