Immunohistochemical analysis of oral lichen-planus-like eruption (LPLE) in graft-versus-host disease (GVHD) using monoclonal antibodies (MoAbs) was performed on five patients after allogeneic bone marrow transplantation (BMT) for leukemia. In the mucosal lesions of LPLE in GVHD, the major population of infiltrated lymphocytes in the areas of upper lamina propria (Lp), basal cell layer (Bc), and epithelium above the basal cell layer (Ep) were T-cells (Leu-1+, Leu-4+) and expressed the phenotype associated with suppressor/cytotoxic T-cells (Leu-2a+) rather than helper/inducer T-cells (Leu-3a+). Some of the infiltrated lymphocytes in the areas of Lp, Bc, and satellite cell necrosis (SCN) bore interleukin-2 (IL-2) receptor. HLA-DR antigen was expressed on keratinocytes in the LPLE lesions. Immunoelectron micrographs showed various degrees of degeneration of keratinocytes to which Leu-2a+ cells attached, whereas those with accidentally attached Leu-3a+ cells preserved normal structures. These findings suggest that cellular immunity mediated by cytotoxic T-cells may play a major role in the pathogenesis of oral LPLE in GVHD.