ObjectiveTo investigate the implication of serum CA19-9 in patients with pulmonary interstitial fibrosis (PF). MethodsOne PF patient was followed up for 5 years, and the levels of serum CA19-9, arterial oxygen pressure and changes of chest CT scanning were recorded and analyzed.Furthermore, another 24 PF patients with the level of serum CA19-9 exceeding the cut-off value (37 U/mL) and 30 healthy controls were recruited, and the pulmonary function was compared between the PF patients and the healthy controls. ResultsDuring the 5-year follow-up, the serum CA19-9 increased gradually from 53.45 U/mL to > 1200 U/mL with progressive reticulation of the lungs proven by the chest CT scanning, and the arterial oxygen pressure in rest without oxygen inhalation decreased from 54 mm Hg to 32 mm Hg.Compared to the healthy controls, the PF patients had significantly higher serum CA19-9 levels [73.8(38.1-1200.0) U/mL vs.6.9(2.0-19.1) U/mL, P < 0.01] and worse diffusion function [55.0(12.7-87.8) mL·min-1·mm Hg-1 vs. 90.4(82.0-134.0) mL·min-1·mm Hg-1, P < 0.05].There was a significantly negative relationship between the elevation of CA19-9 and diffusion function (r=-0.997, P < 0.01). ConclusionSerum CA19-9 may increase in PF patients, which may be an indicator for poor prognosis.