Graft-versus-host disease (GVHD) occurs when lymphoid cells from an immunocompetent donor are introduced into a histoincompatible recipient that is incapable of rejecting them. GVHD is seen most commonly as a complication of therapeutic bone marrow transplantation, and the skin is a primary target organ. As GVHD mimics several important skin diseases both clinically and histologically, it provides a useful model for studying the immunopathologic features of epidermal cell-lymphocyte interactions.