The pathogenesis of Crohn's disease, an intractable inflammatory disease, involves impaired and/or excessive activation of mucosal macrophages and T lymphocytes. B7-1 (CD80) and B7-2 (CD86) molecules are costimulatory molecules that are indispensable to T-cell activation by antigen-presenting cells. To elucidate the roles and characteristics of these antigen-presenting cells in Crohn's disease, in situ localization of B7-1 and B7-2 (in relation to the distribution of T cells) was clarified by light and electron microscopic immunohistochemistry. The results were compared with those from a study of ulcerative colitis. Normal colonic tissue expressed B7-1 or B7-2 only sporadically. In active Crohn's disease, however, an increase in the number of B7-1/B7-2+ cells correlated with an increase in expression of HLA-DR and intercellular adhesion molecule-1. Most B7-1/B7-2+ cells were identified as noncaseating granulomas or as macrophages, which tended to form an aggregate especially in ulcer bases. In active ulcerative colitis, the increase of B7-1/B7-2+ cells was not as prominent as that in Crohn's disease. Double immunohistochemistry revealed a close cellular distribution between noncaseating granulomas and T cells. Immunoelectron microscopy confirmed the expression of B7-1/B7-2 along the plasma membranes of cytoplasmic processes of granuloma cells, where lymphocytes were closely attached. The present study suggested that granuloma formation in Crohn's disease is coupled with antigen presentation via a B7-1/B7-2-CD28 pathway, which may contribute to the pathogenesis of the disease.