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Introduction Juvenile systemic scleroderma (jSSc) is a rare autoimmune disease with high morbidity where interstitial lung disease (ILD), represents a major prognostic determinant. Despite this, ILD prevalence and its relationship to pulmonary function tests (PFTs) in jSSc remain unclear due to small case series and variable definitions. Leveraging the National Registry of Childhood-Onset Scleroderma (NRCOS), the largest single-site jSSc cohort worldwide, we evaluated standardized PFT data and ILD prevalence to better define pulmonary involvement in this population.Material and Methods Registry and medical record review identified all NRCOS participants with available PFTs and computed topography (CT) chest imaging. Lung function values were harmonized to Global Lung Initiative (GLI) reference equations; abnormal was defined as z-score < –1.64. ILD was defined by chest CT findings consistent with ILD (e.g. reticulations, ground glass opacities, in typical distribution). Associations between ILD, PFT parameters, and clinical features were analyzed using bivariate and multivariable models. Statistical analyses were performed in SAS.Results There were 93 jSSc patients in the NRCOS registry with available CT chest and PFT data. jSSc patients had a median age 14.5 years old, 3:1 female-to-male ratio, with prevalent subtypes of diffuse cutaneous and overlap, with average mRSS of 4. ILD was present in 34 (37%) defined by CT imaging. Abnormal forced vital capacity (FVC) and total lung capacity occurred in ~30%, and reduced diffusing capacity (DLCO) in 46%. Although the prevalence of abnormal PFTs (defined by z-score) did not differ between ILD and non-ILD groups ( table 1), median percent predicted FVC (87% vs. 95%, p=0.03) and DLCO (70% vs. 83%, p=0.008) were lower among those with ILD, although many within the normal range. Anti-reflux medication use was more frequent in ILD patients (69% vs. 43%, p=0.02), an association persisting after adjustment for age, sex, disease type, and Scl-70 status (OR 3.6, 95% CI 1.2–10.6, p=0.02).Conclusions ILD affects approximately one-third of children with systemic sclerosis in this largest single-center cohort using standardized definitions of ILD and pulmonary function. While abnormal FVC and DLCO values are common, they did not reliably distinguish ILD status, underscoring their value for longitudinal monitoring rather than diagnosis. The high frequency of abnormal lung function among all jSSc patients suggests that pulmonary involvement is multifactorial, reflecting not only ILD but also respiratory muscle weakness and chest wall restriction. Finally, the association between anti-reflux therapy and ILD identifies a potential modifiable risk factor that merits further prospective evaluation.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.16 Table 1Comparison between children with juvenile-onset systemic sclerosis, with and without interstitial lung disease