Background and objective: Unclassifiable disease in chronic interstitial lung disease (ILD) is a common and challenging problem but has been insufficiently studied. The 2013 update of the international multidisciplinary classification of the idiopathic interstitial pneumonias presented a classification based on observed disease behaviour with the purpose of providing guidance to clinicians in the management of these patients. The aim of this study was to apply the new disease behaviour classification (DBC) and the previously validated ILD-gender age physiology (GAP) score to a cohort of unclassifiable ILD patients and to assess the prognostic value of these two composite approaches. Methods: Unclassifiable ILD patients were identified from the population-based ILD registry at Aarhus University Hospital, Denmark, that includes all ILD patients diagnosed at the department from 2003 to 2009. The DBC was assessed retrospectively based on clinical presentation and diagnostic examinations at presentation and short-term follow-up. Results: Unclassifiable ILD was diagnosed in 24% of the cohort (105 of 431). Survival in unclassifiable ILD was intermediate between idiopathic pulmonary fibrosis (IPF) and non-IPF controls (idiopathic nonspecific interstitial pneumonia (NSIP), hypersensitivity pneumonitis and connective tissue disease-associated ILD). The DBC and the ILD-GAP index were strong individual predictors of outcome in a dichotomized regression model (hazard ratio (HR): 6.3 (95% CI: 1.8-21.2) for the DBC and HR: 6.7 (95% CI: 2.2-20.0) for ILD-GAP). Both approaches remained significant in a common regression model (HR: 4.5 (95% CI: 1.3-15.7) for the DBC and HR: 5.1 (95% CI: 1.3-15.7) for ILD-GAP). Conclusion: The DBC approach showed strong prognostic value in unclassifiable ILD. The DBC and the ILD-GAP were complementary predictors of outcome in unclassifiable ILD.