Sub-maximal exercise gas analysis may be a useful method to assess and track pulmonary arterial hypertension (PAH) severity. The aim of the present study was to develop an algorithm, using exercise gas exchange data, to assess and monitor PAH severity. Forty PAH patients participated in the study completing a range of clinical tests and a novel sub-maximal exercise step test, which lasted 6-minutes and incorporated resting (2-min), exercise (3-min) and recovery (1-min) ventilatory gas analysis. Using gas exchange data, including breathing efficiency (VE/VCO2 slope), end-tidal CO2 (PETCO2), oxygen saturation (SaO2) and oxygen pulse (O2 pulse), a Pulmonary Hypertension Gas Exchange Severity (PH-GXS) score was developed. Patients were re-tested after ~6-months. There was significant separation between healthy controls and moderate PAH (WHO I/II) and more severe PAH (WHO III/IV) patients for VE/VCO2 slope, PETCO2, SaO2 and O2 pulse. The PH-GXS score developed significantly correlated with WHO class (r=0.51), six-minute walk distance (r=−0.59), right ventricle systolic pressure [RVSP] (r=0.49), N-terminal pro-B-type natriuretic peptide [log NT-Pro BNP] (r=0.54) and pulmonary vascular resistance [PVR] (r=0.71). The PH-GXS score remained unchanged in 22 patients re-tested (1.50±0.92 vs. 1.48±0.94), as did WHO class (2.3±0.8 vs. 2.3±0.8) and six-minute walk distance (455±120 vs. 456±103). Small individual changes were observed in the PH-GXS score, with 8 patients improving and 8 deteriorating. In conclusion, the PH-GXS score differentiated between PAH patients and correlated with traditional clinical measures. The PH-GXS score was unchanged in our cohort after 6-months, consistent with traditional clinical metrics, but individual differences were evident. A PH-GXS score may be a useful way to track patient responses to therapy.