Background and Aims: Fibrosis is a major complication of inflammatory bowel disease (IBD), which may be mediated by the intestinal fibroblast. Our aim was to isolate and characterize mucosal fibroblasts from histologically normal intestine (control), ulcerative colitis (UC), inflamed Crohn's disease (CD), and fibrosed CD intestine. Methods: Fibroblasts were characterized by light and electron microscopy and immunohistochemistry. Fibroblast collagen secretion and proliferation were determined by H-3-proline and H-3-thymidine incorporation, and the effects of exposure to interleukin (IL)-1 beta, basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), transforming growth factor (TGF)-beta1, insulinlike growth factor (IGF)-1, and macrophage colony stimulating factor (M-CSF) were determined. Results: No difference in doubling time was observed between the fibroblast populations from UC and CD intestine. All proliferated faster than fibroblasts from control intestine. Collagen secretion from IBD fibroblasts, independent of type, was increased compared with control fibroblasts and PDGF, bFGF, and TGF-beta1-induced colla-en secretion from IBD fibroblasts. Conclusions: These results suggest the presence of an activated subpopulation of fibroblasts in both UC and CD tissue irrespective of the presence of tissue fibrosis or disease type.