Colonic subepithelial myofibroblasts (SEMFs) may play a role in the regulation of a number of epithelial cell functions and in the mucosal repair process. In this study, we evaluated the changes in alpha-smooth muscle actin (SMA)- and vimentin-positive SEMFs in the inflamed mucosa of inflammatory bowel disease (IBD) patients. Tissue samples were surgically obtained from patients with active ulcerative colitis (UC) (n = 5) and active Crohn's disease (CD) (n = 5). Normal intestinal tissues were also obtained (n = 5). The SMA and vimentin expression was evaluated by standard immunohistochemical procedures. In normal intestinal mucosa, SMA- and vimentin-positive SEMFs were located immediately subjacent to the basement membrane, juxtaposed against the bottom site of the epithelial cells. In the inflamed mucosa of active UC patients, there were relatively more SMA-positive cells compared with normal mucosa. In particular, the increase in SMA-positive cells was greatest at the marginal area of deep ulcers of UC patients. In active CD mucosa, SMA-positive cells were increased in all samples, and a marked increase was observed in two samples. The number of SMA-positive SEMFs was relatively higher in CD mucosa than in UC mucosa. An [3H]thymidine incorporation study demonstrated that platelet-derived growth factor (PDGF)-BB, basic fibroblast growth factor (bFGF), and insulin-like growth factor (IGF)-I significantly increased the uptake of [3H]thymidine into isolated SEMFs. In particular, PDGF had a strong stimulatory effect. We concluded that colonic SEMFs may play an important role in the repair process of IBD.