Patients with pulmonary arterial hypertension (PAH) usually show improvements in symptoms, exercise capacity, and hemodynamics after treatment with approved medical therapies. This study sought to determine whether improvement in right-sided cardiac function measured using cardiac magnetic resonance imaging would also be seen and whether these changes would correlate with improvement in exercise capacity. Sixteen patients with PAH underwent evaluation at baseline and after 12 months of treatment with bosentan. After treatment, cardiac index, pulmonary vascular resistance, and 6-minute walk distance improved, and there was a trend toward improvement in right ventricular (RV) stroke volume (70 ± 27 to 81 ± 30 ml; p = 0.08), but no change in RV ejection fraction (RVEF) or RV end-diastolic volume. Six-minute walk distance improved by 59 m (p <0.05) in the overall cohort and improved more in patients in whom RVEF increased compared with those with stable or decreased RVEF (+98 vs −37 m, respectively; p = 0.01). Three patients died during follow-up, and these patients had significantly lower RVEF and left ventricular end-diastolic volume indexes than surviving patients. In conclusion, these results suggest that cardiac magnetic resonance imaging may have value in determining response to therapy and prognosis in patients with PAH.