Adhesion of leukocytes to vascular endothelium is a necessary step leading to the migration of cells into underlying tissues. Vascular adhesion molecules regulate this process and may play an important role in graft rejection. Immunocytochemical studies have been used to investigate the expression of vascular adhesion molecules (ICAM-1, PEC AM, VCAM-1, and ELAM-1) in normal donor heart (n = 15) and myocardial biopsies from heart transplant patients with acute rejection (n = 15). Sections were also stained with antibodies against endothelium, leukocytes, MHC antigens, and markers of cell activation. In donor heart EN4, vWF, ICAM-1, PECAM, MHC class I—and, to a lesser extent, VCAM-1 and DR antigen—are expressed on arterioles and venules, whereas ELAM-1 and Pal-E are restricted to venules. Expression of Pal-E, VCAM-1, ICAM-1, and DR antigen was increased during rejection. Capillary endothelium normally expresses EN4, ICAM-1, PECAM, MHC class I, and DR antigen but little, if any, VCAM-1 or ELAM-1. During rejection, however, there is an increased expression of all adhesion molecules. This is paralleled by an increased expression of vWF by capillary endothelium. In addition, ICAM-1 like MHC class I antigen is induced on the myocardial membrane and intercalating discs. Endocardium from donor heart expresses EN4, vWF, PECAM, MHC class I, and sometimes Pal-E and ICAM-1, but very little VCAM-1, ELAM-1 or DR antigen. There is an increased expression of Pal-E, ICAM-1, VCAM-1, and DR antigen on endocardium from rejecting heart biopsies. Proliferating Ki-67+ cells and activated T cells expressing the receptor for IL-2 were also found in biopsies during rejection episodes.