Pulmonary vascular resistance (PVR)>3 WU is a criterion of the haemodynamic definition of pulmonary arterial hypertension (PAH). However, this cut-off is conservative and arbitrarily defined. Data is lacking on the natural history, response to therapy and survival of patients diagnosed with precapillary pulmonary hypertension with mild or borderline elevation of PVR.In Australia, PAH therapy could be prescribed solely on mean pulmonary artery pressure (mPAP) and pulmonary artery wedge pressure (PAWP) criteria. Using the Australian and New Zealand Pulmonary Hypertension Registry, we aimed to study a population diagnosed with PAH between Jan 2004 and Dec 2017 with the pre-defined haemodynamic characteristics of mPAP≥25 mmHg, PAWP≤15 mmHg and PVR<3 WU.Eighty-two patients met the pre-defined haemodynamic inclusion criteria (mean age 63±11 years; 67 females). Underlying aetiologies included idiopathic disease(n=39), connective tissue disease(n=42) and HIV infection(n=1). At diagnosis, mPAP was 27 mmHg(IQR25-30), PAWP 13 mmHg(IQR11-14) and PVR 2.2 WU(IQR1.9-2.7). Baseline 6MWD was 352 m(IQR280-416) and 77% were in NYHA 3 or 4 functional class. All patients were commenced on initial monotherapy with an endothelin receptor antagonist(n=66) or phosphodiesterase-5 inhibitor(n=16). At first re-evaluation, 6MWD increased by 46 m(IQR7-96) and 35% demonstrated improvement in NYHA functional class. After a median follow-up of 65 months (IQR32-101), 18/82(22.0%) had died, with estimated 1-yr and 5-yr survivals of 98% and 84%, respectively. Death attributed to PAH occurred in 6/18(33.3%) of these patients (7% of total cohort).Patients with precapillary PH and "borderline" PVR falling outside the current definition have adverse outcomes. Such patients appear to respond to PAH therapy however this requires further study in randomised trials.