An immunohistological study of epidermal keratinocytes for the intercellular adhesion molecule, ICAM-1 (CD54), was undertaken on skin biopsies from allogeneic bone marrow transplant recipients. In control biopsies from normal donors and patients prior to transplantation, staining was weak and confined to relatively few cells. After transplantation there was a significant increase in both the number of positive cells and their staining intensity in biopsies showing histological evidence of GVHD but not in those exhibiting normal appearances or epidermal abnormalities that could be attributed to cytotoxic drugs or irradiation. There was a strong positive correlation between ICAM-1 and HLA-DR expression by keratinocytes. All cases exhibiting increased ICAM-1 also exhibited an increase in HLA-DR antigens. The converse was not true, however, as 6 biopsies exhibited HLA-DR positivity without detectable increases in ICAM-1. Both ICAM-1 and HLA-DR antigen synthesis may be stimulated by local cytokine release following interactions between donor and recipient cells in the early posttransplant period. Our present findings suggest that immunostaining for ICAM-1 has little value in the early diagnosis of cutaneous GVHD but further, more detailed prospective studies would be of value.