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Introduction: Pulmonary arterial hypertension is defined at cardiac catheterization as a mPAP≥ 25 mmHg and a pulmonary arterial wedge pressure (PAWP) ≤15mmHg. The presence of cardiac comorbidities is associated with a poorer response to therapy in patients with PAH and left ventricular diastolic dysfunction may exist even in the presence of a normal resting PAWP. The aim of this study was to use left atrial volume as a surrogate of left ventricular diastolic dysfunction in patients with normal PAWP and assess its impact on treatment response in PAH. Methods: We studied 85 treatment naive patients with PAH with an MRI at baseline and follow up at 1 year (median 1year IQR 0.8). Patients were grouped by LA volume threshold of 34ml/m2 based on established literature. Changes in biventricular volume, mass and function were compared. Results: Compared to patients with elevated LA volume (n=31), those with normal LA volume (n=54) had greater treatment response with increase in change in RV ejection fraction, LV stroke volume, LV end diastolic volume and diastolic interventricular septal angle (p=0.032, p=0.001, p=0.001 and p=0.025 respectively). Patients with normal LA volume had a significant improvement in RV ejection fraction between baseline and follow up (mean increase 9%, p Conclusion: This study shows that in patients meeting the haemodynamic criteria for PAH, patients with an elevated LA volume had poor response to PAH therapy. Further work to better characterize this PAH phenotype and recognize the importance of comorbidities in predicting response to therapy is required.