Assessment of cardiac output is an important part of the management of patients with pulmonary hypertension. The accuracy of the thermodilution technique in patients with low cardiac output or severe tricuspid regurgitation has been questioned. To address this issue, we simultaneously compared 105 cardiac output measurements by the Fick method and thermodilution in 35 patients with pulmonary hypertension. Moreover, we evaluated the acetylene rebreathing technique, a noninvasive method of determining cardiac output. The mean difference +/- 95% limit of agreement between thermodilution and the Fick method was +0.01 +/- 1.1 L/min. The mean difference +/- 95% limit of agreement between acetylene rebreathing and the Fick method was -0.23 +/- 1.14 L/min. Neither the mean agreement nor the 95% limits of agreement of both thermodilution and acetylene rebreathing with the Fick method were affected by the presence of low cardiac output or severe tricuspid regurgitation. We conclude that thermodilution and acetylene rebreathing are useful tools for assessing cardiac output in patients with pulmonary hypertension, even in the presence of low cardiac output or severe tricuspid regurgitation.