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Objective: We propose a non-invasive method for diagnosing post-capillary pulmonary hypertension (PH group 2). We evaluated pulmonary capillary wedge pressure (PCWP) by studying the left atrium (LA) on thoracic ECG-gated CT compared with right heart catheterisation (RHC)., Methods: We retrospectively studied 54 patients with suspected PH or followed for PH who underwent thoracic ECG-gated CT and RHC within 15 days. The diagnostic accuracy of CT morphological and functional data of the LA for the detection of PCWP >15 mmHg, evaluated by two independent readers, was assessed using correlation and receiver-operating characteristic (ROC) analysis., Results: Interobserver agreement was high (r = 0.97-0.99). Correlations were found between PCWP and the morphological criteria of the LA such as anteroposterior diameter at 0 % of the R-R interval (r = 0.70, P ≤ 0.001) as well as at 40 % (r = 0.69, P ≤ 0.001). ROC curves constructed with a threshold value of PCWP > 15 mmHg showed an area under the curve between 0.88 and 0.91. Significant correlations were found between PCWP and functional criteria of the LA, including distensibility (r = -0.49, P ≤ 0.001) and ejection fraction (r = -0.58, P ≤ 0.001)., Conclusion: Thoracic ECG-gated CT in a PH workup helps distinguish between pre- and post-capillary PH., Key Points: • Computed tomography may help differentiate the various types of pulmonary hypertension (PH). • Post-capillary PH group 2 is due to left heart disease. • Right heart catheterisation is used to separate pre- and post-capillary PH. • Left atrium anteroposterior diameter measured on CT is of value. • ECG-gated CT helps clinicians to assess patients with PH non-invasively.