BACKGROUND AND OBJECTIVE: Patients with fibrotic interstitial lung disease (FILD) have impaired health status. The simple questionnaire in the COPD assessment test (CAT) has been validated for idiopathic pulmonary fibrosis (IPF) and connective tissue disease-related interstitial lung disease (CTD-ILD), but no or limited data exist for patients with FILD as a whole. The aim of this study was to evaluate the reliability, repeatability and responsiveness of the CAT, and estimate the minimal clinically important difference (MCID) in patients with FILD. METHODS: This study was a retrospective chart review of 358 consecutive patients with FILD including 131 with IPF, who underwent clinical assessment over 6-12 month intervals. We assessed the cross-sectional and longitudinal validity of the CAT. MCID was estimated using distribution methods and anchor methods with mean change and regression models. RESULTS: Internal consistency (Cronbach's alpha = 0.898) and repeatability (intraclass correlation coefficient [ICC] = 0.865) for the CAT score was acceptable. A cross-sectional study showed constructive validity. Changes in the CAT over 6-12 months were significantly associated with change in anchors including physiological function, exercise capacity, and dyspnea regardless of IPF diagnosis. The estimated MCIDs of the CAT for the deterioration and improvement directions were at least +5 and at least -3 points, respectively. CONCLUSIONS: The CAT is a reliable, responsive and clinically relevant instrument for assessing health status in patients with FILD.