During the progress of IPF, (myo)fibroblasts secrete a large amount of extracellular matrix, including collagen I. In order to determine the effect of IL-9 on collagen I secretion by fibroblasts, we stimulated fibroblasts with IL-9 alone or in the presence of TGF-β for 24 h or 48 h and then detected the Col1a1 expression by real-time qPCR. The gene discussed is IL9; the disease is idiopathic pulmonary fibrosis.