A growing body of evidence suggests that exercise pulmonary hypertension (ePH) is an early form of pulmonary arterial hypertension (PAH). Identifying the disease at an early, potentially more responsive phase, and initiating treatment may improve functional status and prevent progression to severe forms of PAH. This was a single-center, open-label six-month treatment trial to evaluate the effect of ambrisentan on pulmonary hemodynamics and exercise capacity in ePH utilizing invasive cardiopulmonary exercise testing (iCPET). After six months of treatment with ambrisentan, patients repeated iCPET; exercise capacity, symptoms, and pulmonary hemodynamics were reassessed. Twenty-two of 30 patients completed the treatment phase and repeat iCPET. After six months of treatment there was a significant decline in peak exercise mPAP (−5.2 ± 5.6 mmHg, P = 0.001), TPG (−7.1 ± 8.0 mmHg, P = 0.001), PVR (−0.9 ± 0.7 Woods units, P = 0.0002), and Ca-vO2 (−1.8 ± 2.3 mL/dL, P = 0.0002), with significant increases in peak PCWP (+2.9 ± 5.6 mmHg, P = 0.02), PVC (+0.8 ± 1.4 mL/mmHg, P = 0.03), and CO (+2.3 ± 1.4 L/min, P = 0.0001). A trend toward increased VO2max (+4.4 ± 2.6% predicted, P = 0.07) was observed. In addition, there were improvements in 6MWD and WHO FC after 24 weeks. Our findings suggest that treatment of ePH with ambrisentan results in improved pulmonary hemodynamics and functional status over a six-month period. Treatment of ePH may prevent the progression of vascular remodeling and development of established PAH.