Context: CC chemokine ligand 18 (CCL18) is suggested to play a role in the development of pulmonary fibrosis. Macrophages are thought to be the main source of CCL18, and the effect of pirfenidone, an anti-fibrotic agent for idiopathic pulmonary fibrosis, on the expression of CCL18 in macrophages warrants investigation.Objective: The purpose of this study was to investigate the effect of pirfenidone on the expression of CCL18 in macrophages.Materials and methods: U937 cells were differentiated into macrophages by phorbol myristate acetate and then stimulated with recombinant IL-4 to induce the production of CCL18. The cells were treated with pirfenidone, and the mRNA and protein levels for CCL18 were measured by a reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. The effects of pirfenidone on the IL-4 receptor (IL-4R) expression and STAT6 activation were investigated and on the JAK kinase activity were measured using the Z-LYTE kinase assay.Results: Pirfenidone significantly suppressed the expression of CCL18 when the cells were treated with concentrations of 50-250g/mL. Pirfenidone did not affect the expression of the IL-4R components. The selective STAT6 inhibitor AS1517499 suppressed CCL18 expression. Both AS1517499 and pirfenidone suppressed STAT6 phosphorylation (p<.05), although the effect of pirfenidone was less marked than that of AS1517499. The Z-LYTE kinase assay showed a reduction in the activities of JAK1, JAK3 and TYK2 by pirfenidone.Conclusion: Pirfenidone suppresses CCL18 expression in macrophages and this effect is thought to be attributed partly to the inhibition of STAT6 phosphorylation.