Abstract: The PDE4-inhibitor of formula A? for use in a method for the treatment of a progressive fibrosing interstitial lung disease (PF-ILD), preferably IPF, in a patient comprising the following steps: a) measuring or having measured the concentration, expression level or activity of one or more biomarkers selected from the group consisting of Krebs von den Lungen protein (KL-6), Pulmonary Surfactant Protein D (SP-D), Matrilysin (MMP7), CA-125 (also named MUC-16), CA19-9, E-selectin, sICAM-1, Stromelysin (MMP3), osteopontin (OPN), connective tissue growth factor (CTGF), Cartilage oligomeric matrix protein (COMP), prostasin, von-Willebrand-Faktor (vWF), and C-reactive protein (CRP) in the serum or plasma of a blood sample obtained from said patient, b) comparing or having compared the concentration, expression level or activity of the one or more biomarkers as listed in step a) in said patient's blood serum or blood plasma sample to a reference concentration, expression level or activity of the respective one