Phenotypic characteristics of vascular endothelial cells of the colonic mucosa in patients with ulcerative colitis and healthy controls were studied with immunoperoxidase staining by light and electron microscopy. The cells could be classified into two groups according to their phenotypes; one was positive for von Willebrand factor and the other had an antigen detected by a monoclonal antibody, OKM5. The endothelial cells positive for von Willebrand factor were usually in relatively large blood vessels, and OKM5-positive cells were mostly located in small capillaries along the glandular epithelium. OKM5-positive endothelial cells also expressed HLA-DR and interleukin-1 (IL-1). In patients with ulcerative colitis, OKM5-positive endothelial cells and spindle-shaped cells that might be precursors of endothelial cells were more numerous in the lamina propria than in the other subjects. Thus, OKM5-positive endothelial cells may be important as antigen-presenting cells and immunoregulatory cells in the intramucosal immune system. Furthermore, colonic epithelial cells in patients with ulcerative colitis synthesized HLA-DR and IL-1, and may have a close relation to immune responses, such as antigen processing and presentation to immunocompetent cells. It was suggested that these cells have a close relation to the pathogenesis of the impaired immune responses in situ in ulcerative colitis.