This study sought to assess the safety of cardiopulmonary stress testing in 40 children with pulmonary arterial hypertension and to compare exercise responses in this patient cohort with those of a healthy control population. Patients with pulmonary hypertension had significant impairment in aerobic capacity, with a peak oxygen consumption of 20.7 +/- 6.9 versus 35.5 +/- 7.4 ml/kg/min in healthy controls (p <0.0001). Peak oxygen consumption was strongly correlated with invasive measures of disease severity, including pulmonary vascular resistance index (r = -0.6, p = 0.006). Exercise testing can be performed safely in this group of patients and can yield valuable clinical information.