This study aimed to determine the predictive factors for the development of pulmonary fibrosis by combining thorax CT findings at the time of hospitalization, serum prolidase activity (SPA), MMP-1, MMP-7, TGF-β values, laboratory findings, clinical–radiological features (follow-up thorax CT requested only in the case of clinical necessity) at the 12th week routine clinical controls, and demographic characteristics of patients who were treated for COVID-19 in our respiratory medicine service, had moderate to severe disease, and were discharged. This evidence concerns the gene PEPD and pulmonary fibrosis.