The B7/CD28 pathway is essential for initiating antigen-specific T-cell activation. LFA-1 (CD11a/CD18) is required for sufficient migration into inflammatory tissue. The aim of this study was to evaluate the role of B7 and LFA-1 in inflammatory bowel disease. Immunohistological single and double staining (PAP/APAAP) with monoclonal antibodies against HLA I/II, CD4, CD8, CD28, B7-1, B7-2, LFA-1 and CD68 were performed in tissue samples from patients with crohn's disease (n = 15), ulcerative colitis (n = 14), colorectal carcinoma (n = 5) and FAP (n = 3). The expression of B7-1 and B7-2 was generally much higher in ulcerative colitis and crohn's disease than in colorectal carcinoma and FAP. In crohn's disease multinucleated gigant cells in the granulomas express B7-1 and B7-2. Double staining showed a higher B7-1/B7-2 coexpression for CD4+ than for CD8+ T cells. In colitis ulcerosa and crohn's disease LFA-1 positive leucocytes showed a high coexpression of B7-1 and B7-2 in contrast to CD68 positive macrophages. These data suggest that overexpression of B7-1 and B7-2 on LFA-1 positive Leucocyts seems to play an important role in the pathogenesis of inflammatory bowel disease.