Careful monitoring of disease progression is vital to ensuring that patients with pulmonary arterial hypertension receive maximal therapy before the onset of overt right-sided heart failure. Routine follow-up includes the evaluation of symptoms, functional class, and exercise capacity and assessment of pulmonary pressures and right ventricular (RV) function. Transthoracic echocardiography (TTE) offers a noninvasive and fairly reliable technique for monitoring pulmonary artery pressure (PAP) and structural changes of the right side of the heart. However, TTE does not reliably assess cardiac output, right-sided filling pressures, or pulmonary venous pressure. Pulmonary artery catheterization may be particularly useful in patients who have inconsistent findings, such as a reduction in PAP measured by TTE in the presence of worsening symptoms or other signs of disease progression. An increase in RV end-diastolic pressure, usually above 10 mm Hg, is a concern and warrants consideration of additional therapy even if other hemodynamic and clinical parameters are unchanged. (J Respir Dis. 2009;30(1-2)