BACKGROUND: Tumor necrosis factor-alpha (TNF-alpha), may be involved in the pathogenesis of inflammatory bowel diseases (IBDs). The aim of this study was to evaluate the effect of TNF-alpha on the inflammatory activity of IBD. METHODS: TNF-alpha mRNA expression in intestinal mucosal biopsy specimens from IBD patients [ulcerative colitis (UC), n = 54; and Crohn's disease (CD), n = 11] was analyzed using a competitive polymerase chain reaction. The degree of macrophage infiltration was analyzed by immunohistochemistry, using an antihuman CD68 antibody. RESULTS: TNF-alpha mRNA expression was increased in UC patients, corresponding to the inflammatory activity. However, in CD, TNF-alpha mRNA expression was not correlated with the endoscopic findings. CONCLUSIONS: We clarified that TNF-alpha mRNA expression was responsible for the inflammatory activity in UC. However, TNF-alpha mRNA expression was not correlated with the mucosal injury in CD.