The aim of the present study is to clarify in situ expression of platelet-derived growth factor (PDGF) and its receptors in different phases of inflammatory bowel disease (IBD). Tissues samples were obtained from 20 patients with ulcerative colitis (UC) and 29 with Crohn's disease (CD) at surgery. In situ hybridization and immunohistochemistry on frozen sections were performed for PDGF-A and -B and its α and β receptors (αR and βR). The area of active inflammation was infiltrated by abundant polymorphonuclear and mononuclear leukocytes, of which the latter expressed mRNA and proteins of PDGF-A, -B, and -αR and mRNA for PDGF-βR. The area of active fibrosis, characterized by activated fibroblasts/myofibroblasts, was juxtaposed to ulceration, which is induced as a repair process to tissue destruction. In these areas, activated fibroblasts/myofibroblasts were positive for mRNA and protein of PDGF-A, -B, -αR, and -βR. The expression of PDGF-A, -B, and -αR declined significantly in the scar area. Our results suggest that PDGF is not only important as an inducer of fibrosis in the repair phase but also it is involved in the active inflammatory phase possibly as a chemoattractant for mononuclear inflammatory cells.