Idiopathic, nonspecific interstitial pneumonia (NSIP) is most often associated with various clinical disorders, including connective tissue diseases (CTDs) and chronic hypersensitivity pneumonitis (cHP). Emerging evidence also suggests that "idiopathic" NSIP may be the lung manifestation of undifferentiated CTD (UCTD). However, whether or not NSIP outcome is influenced by the underlying cause remains uncertain.This retrospective study included 127 biopsy-proven NSIP patients (65 women, mean±SD age 55±12 years). Survivals were estimated using a Kaplan-Meier curve and compared using the log-rank test. Multivariate analyses were based on a Cox model. 15 (11.8%) patients had cHP, 29 (22.8%) had CTD, 32 (25.2%) satisfied the Kinder criteria for UCTD and 51 (40.1%) had idiopathic NSIP. At the end of follow-up (mean±SD 64±54 months), a difference in survival was observed between aetiological groups ( p=0.002). Survival was better for UCTD than for idiopathic NSIP ( p=0.020) and similar to that observed for CTD. cHP survival tended to be poorer than that of idiopathic NSIP ( p=0.087) and was an independent predictor of mortality (hazard ratio 2.17, 95% CI 1. 05-4.47; p=0.035).NSIP outcome is influenced by its cause. cHP exhibits the highest mortality. UCTD does not differ from CTD supporting the concept of autoimmune NSIP, with a prognosis that is better than that of idiopathic NSIP. @ERSpublications NSIP patients should be investigated for the presence of an underlying cause, which significantly impacts survival

Non-specific interstitial pneumonia subdivision into pathological subgroups is clinically relevant from a prognostic and causal perspective.