Summary Cathelicidin peptide LL-37 plays an important role in the early host response against invading pathogens via its broad-spectrum anti-microbial activity. In this study, we investigated LL-37 expression in the inflamed mucosa of inflammatory bowel disease (IBD) patients. Furthermore, the regulatory mechanism of LL-37 induction was investigated in human colonic subepithelial myofibroblasts (SEMFs). LL-37 mRNA expression and protein secretion were analysed using real-time polymerase chain reaction and enzyme-linked immunosorbent assay, respectively. Intracellular signalling pathways were analysed using immunoblotting and specific small interference RNA (siRNA). The expression of LL-37 mRNA was increased significantly in the inflamed mucosa of ulcerative colitis and Crohn's disease. The Toll-like receptor (TLR)-3 ligand, polyinosinic-polycytidylic acid (poly(I:C), induced LL-37 mRNA expression and stimulated LL-37 secretion in colonic SEMFs. The transfection of siRNAs specific for intracellular signalling proteins [Toll/IL-1R domain-containing adaptor-inducing interferon (IFN) (TRIF), tumour necrosis factor receptor-associated factor (TRAF)6, transforming growth factor β-activated kinase (TAK)1] suppressed the poly(I:C)-induced LL-37 mRNA expression significantly. Poly(I:C)-induced phosphorylation of mitogen-activated protein kinases (MAPKs) and activated nuclear factor kappa B (NF-κB) and activating factor protein (AP)-1. siRNAs specific for NF-κB and c-Jun inhibited poly(I:C)-induced LL-37 mRNA expression. LL-37 suppressed lipopolysaccharide (LPS)-induced interleukin (IL)-6 and IL-8 expression significantly in colonic SEMFs. The expression of LL-37 was up-regulated in the inflamed mucosa of IBD patients. LL-37 was induced by TLR-3 stimulation and exhibited an anti-microbial effect via interaction with lipopolysaccharide (LPS).