Immunohistochemical techniques were used to investigate the pathogenesis of obliterative arteriopathy, a major obstacle to long-term solid organ allograft survival. T-lymphocytes, macrophages, and proliferating smooth muscle cells made up most of the thickened intima. More importantly, S100-protein-positive dendritic cells were also present in the intima, especially during active inflammation and smooth muscle cell proliferation. These are phenotypic characteristics of tissue "dendritic" cells, pivotal accessory cells in T-dependent immune reactions. Their localisation in the arterial wall signals the presence of an ongoing immunological reaction directed at native constituents of the artery or at exogenous antigens which permeate the damaged vessel wall.