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Background Current ERS/ESC guidelines use a pulmonary vascular resistance (PVR) of 5WU to distinguish severe from non-severe PH associated with lung disease (Group 3). However, there are limited data on how best to risk stratify patients.Aims and Objectives To assess the prognostic value of non-invasive and invasive testing in Group 3 PH.Methods Consecutive patients from the ASPIRE registry with group 3 PH diagnosed between 2007 and 2023 were included. Patients underwent a comprehensive assessment including pulmonary function and exercise testing, CT imaging, cardiac MRI and right heart catheterisation. Prognostic value was assessed using Kaplan-Meier (KM) and multivariable Cox regression analyses.Results 774 patients, 52% male, mean age 67(12) yrs were included. 48% had COPD/emphysema, 27% ILD, 14% CPFE, and 11% non-parenchymal restriction. There were 588 deaths with a median survival of 24 months (10–55). Predictive variables from univariate analysis were incorporated into a multivariable model and refined. Age, male sex, DLCO%predicted (HR: 0.98, 95% CI: 0.97–0.99, p<0.001), walk distance (HR 0.98 95% CI 0.97–0.99 p=0.006), RVEF (HR 0.98 95% CI 0.97–1 p=0.04) and PVR (HR 1.06 95% CI 1.02–1.11 p=0.002) were independently associated with survival. KM analysis stratifying patients by PVR > or ≤5WU and DLCO > or ≤ 28% (median) demonstrated that low DLCO further differentiated survival within both high and low PVR groups. DLCO also predicted survival in all Group 3 subgroups.Abstract S82 Figure 1Conclusion DLCO is a strong, independent predictor of mortality in group 3 PH. When used in combination with PVR it improves mortality prediction in all subgroups.