Introduction: The INMARK trial investigated effects of nintedanib on blood biomarkers in subjects with IPF. Aim: To investigate the effect of nintedanib on markers of epithelial damage. Methods: Subjects (n=346) with IPF and FVC ≥80% predicted were randomised 1:2 to receive nintedanib 150 mg bid or placebo double-blind for 12 weeks, followed by open-label nintedanib for 40 weeks. Blood samples were taken at weeks 4, 8, 12, 16, 20, 24, 36, and 52. We assessed the rate of change (slope) in log10-transformed CA-125 and CA19-9 (two markers of epithelial damage) from baseline to week 12 using random coefficient regression and changes in log10-transformed CA-125 and CA19-9 over 52 weeks using a mixed model for repeated measures. Results: The adjusted rate of change in CA-125 levels from baseline to week 12 was significantly different for nintedanib vs placebo (difference -0.046 U/mL/month [95% CI ‑0.056, -0.035]; p Conclusion: CA-125 may be a biomarker of response to nintedanib in patients with IPF.