Pneumonitis followed by lung fibrosis is a frequent complication of radiation therapy of chest tumors. A hallmark of these fibrotic lesions is the excessive production and accumulation of extracellular matrix proteins such as type I collagen. In addition to TGF-β1, IL-4 has been recognized as a potent inducer of collagen gene synthesis in fibroblasts. In this study, we analyzed the regulation of the α1(I) procollagen (COL1A1) promoter and the α2(I) procollagen (COL1A2) promoter by IL-4 in normal human lung fibroblasts. We provide evidence that the IL-4-induced transcriptional activator STAT6 binds to various sequences within the COL1A1 and COL1A2 promoter. The regulatory function of these regions was tested by reporter gene analysis using 5′ deletions of the COL1A1 and COL1A2 promoter fused to the luciferase gene. Interleukin-4 treatment of human fibroblasts transiently transfected with COL1A1 promoter deletion constructs resulted in luciferase activity exceeding that of untreated fibroblasts by 25%, while luciferase activity driven by the COL1A2 promoter was enhanced by about 70% upon IL-4 treatment. A combined action of SP1, NFκB, and STAT6 essentially contributes to the IL-4 mediated COL1A2 gene activation. An AP2 site adjacent to the reverse orientated STAT6 consensus motif TTC N3/4 GCT is located within 205 bases from the transcription start site and seems to support the moderate IL-4-induced COL1A1 gene activation. Interferon-γ downregulation of transcription is mainly seen with the COL1A1 promoter. J. Cell. Physiol. 198: 248–258, 2004© 2003 Wiley-Liss, Inc.