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Introduction Pulmonary arterial hypertension (PAH) develops in 6–12% of systemic sclerosis (SSc) and is a major cause of morbidity and mortality. Current guidelines recommend risk-adapted therapy, with initial double-combination therapy for low-/intermediate-risk and triple therapy for high-risk patients. Most evidence derives from idiopathic PAH studies, whereas SSc-PAH patients have worse prognosis and less favourable treatment response. As such, robust long-term data in SSc-PAH are lacking. This gap is particularly evident in patients with milder haemodynamic impairment (mPAP 21-24 mmHg or PVR 2-3 WU), who remain at risk for adverse outcomes but lack evidence-based treatment guidance. We therefore aimed to evaluate the impact of initial therapy on mortality and PAH progression across haemodynamic thresholds and risk strata.Material and Methods We included incident, treatment-naïve SSc-PAH patients (2001–2021) from the EUSTAR database fulfilling the 2022 haemodynamic definition of PAH (EUSTAR Project CP122). Patients with severe ILD were excluded. Haemodynamics were classified as higher (mPAP >=25 mmHg, PVR >3 WU) or lower (mPAP 21-24 mmHg and/or PVR 2-3 WU). Risk was stratified using the 2022 ESC/ERS four-strata model. Initial therapy was PAH treatment within four months of diagnosis and continued >=3 months. Untreated patients served as comparators. Outcomes were all-cause mortality and PAH progression within 36 months, assessed by multivariable Cox regression adjusting for age, sex, DLCO, haemodynamics, and risk group.Results Of 301 patients, 170 (57%) received initial therapy (108 monotherapy, 62 combination); 131 (43%) were untreated. Treated patients had worse baseline haemodynamics and risk profiles ( table 1). Over a median 3.9 years (Q1–Q3: 1.6–6.3), 125 (42%) died. Among 227 patients with progression data, 85 (37%) progressed within 36 months (figure 1A). Adjusted analyses showed initial therapy was associated with reduced mortality and PAH progression, independent of haemodynamic thresholds and risk stratification (figure 1B and C). In the lower haemodynamic threshold group (n=83), 18 (22%) received initial therapy (16 monotherapy, 2 combination). Over 5.1 years (Q1–Q3: 2.5–7.9), 20 (24%) died. Adjusted for risk stratification, results suggested reduced mortality with initial therapy (HR 0.36, 95% CI 0.08–1.67), though not statistically significant (figure 1D).Conclusions Initial therapy was associated with improved survival and reduced PAH progression in SSc-PAH, with consistent effects across haemodynamic thresholds and risk strata. These findings support guideline-recommended early intervention, highlight the importance of high-quality observational data in rare diseases, and underscore the need for RCTs to clarify treatment effects in patients with milder haemodynamic impairment.Abstract OC.03 Figure 1Abstract OC.03 Table 1Comparison of base line characteristics of treated patients and the untreated comparator group