The lymphocytic subtypes effecting allogeneic graft-versus-host disease (GVHD) are unknown. We studied 35 skin biopsy specimens from 19 women transplanted with bone marrow from men for patterns and time course of infiltration of the skin by Y chromosome-bearing lymphocytes using in situ hybridization. Immunophenotypic analysis was performed on serial sections. Significant numbers of donor cells were first observed by day 13 after bone marrow transplantation (BMT), although a few cells were noted at earlier time points. The quantity of donor lymphocytes in the dermis correlated with the diagnosis of GVHD. For specimens with grade 1 features, only rare cells bore the Y chromosome, whereas the majority of lymphocytes in grade 2 tissues, whether heavily inflamed or not, contained the Y chromosome. These lymphocytes were predominantly CD4+ with fewer CD8+ and CD56+ cells in the dermis and epidermis. No concentration of a specific subtype in the epidermal compartment was observed. These data do not support the observation that a cutaneous graft-versus-host reaction (GVHR) is mediated primarily by CD8+ lymphocytes. Several effector cell populations may mediate a cutaneous GVHR with further variation over time and in BMTs between different individuals.