The presence of an erythematous skin rash and hemorrhagic complications in acute graft-versus-host disease (GVHD) suggest that the vasculature may be involved in the immunopathologic process. We reviewed endothelial and vascular histopathologic changes on light microscopy and on immunoperoxidase stained sections of skin biopsies obtained from 41 HLA-identical allogeneic marrow transplant recipients with at least grade 2 GVHD. Biopsies taken from 14 allogeneic HLA-identical bone marrow transplant recipients who never developed GVHD were used as controls. Sections were evaluated for evidence of immunologic vascular injury using the rank file analysis of histologic features, expression of HLA-DR antigen, and the distribution of fibrin and factor VIII-related antigen (F VIII RAg). Patients with acute GVHD had significantly greater intimal lymphocytic infiltrates, perivascular nuclear dust deposition, perivascular F VIII Rag extravasation and deposition and vascular proliferation than controls. We find significantly greater endothelial injury in GVHD patients, which may represent primary immunologic injury to the vasculature. The clinical findings in acute GVHD probably result from cumulative endothelial as well as epithelial injury.