Background: The purpose of this study was to determine whether an implantable hemodynamic monitor (IHM) could be used to judge the response of pulmonary arterial hypertension (PAH) patients to changes in therapy. Methods: A prospective, non-randomized, multi-center study evaluated physical examination, functional class, echocardiography, brain natriuretic peptide (BNP) levels, exercise capacity assessed by 6-minute walk and cardiopulmonary exercise tests, and quality of life at baseline and at 12 weeks. IHM measurements were continuously available to clinicians between clinic visits. Based on a priori, pre-specified analyses, the relationships between hemodynamic values, PAH treatments and clinical parameters were tracked in an observational fashion. Results: Twenty-four PAH patients underwent IHM implantation prior to a change in PAH therapy. IHM data identified 13 of the 15 patients who improved their 6-minute walk distance by >30 m at 12 weeks (+48 ± 65 m, p < 0.05), whereas the others walked less (-78 ± 115 m, not statistically significant). In addition, peak Vo2, BNP levels and Minnesota Living with Heart Failure Questionnaire scores only improved in the former group. The change in mean pulmonary artery pressure correlated with the change in 6-minute walk distance at 12 weeks (r = -0.71, p < 0.001). Device-related adverse events were comparable to those known to occur with a pacemaker-like device. Conclusions: Changes in ambulatory continuous hemodynamic measurements predicted changes in 6-minute walk distance after the start or addition of PAH therapy. The IHM also identified patients who had improved exercise tolerance, BNP levels and quality of life. The IHM appeared to be well tolerated and allowed rapid hemodynamic feedback between clinic visits. © 2008 International Society for Heart and Lung Transplantation.