AIMS--To define the immunopathological profile of transfusion associated graft versus host disease (TA-GvHD) to elucidate its pathophysiology and to determine if any features are of diagnostic value. METHODS--Nine patients (age range 14-61 years) who developed histologically confirmed TA-GvHD between 1989 and 1992 were studied. Immunohistochemical analysis of frozen and formalin fixed skin biopsy tissue was performed. Sections were stained with antibodies to CD3, CD8, CD4 and HLA-DR, using a routine streptavidin-biotin technique with standard diaminobenzidine development. RESULTS--All biopsy specimens showed aberrant positive expression of HLA-DR by epidermal keratinocytes. In four patients, all of whom died, HLA-DR was diffusely expressed throughout the epidermis; in the other five cases keratinocyte expression of HLA-DR was more focal. In all biopsy specimens T cells had infiltrated the dermis and epidermis. In all nine cases CD4+ T helper/inducer cells were the predominant T cells. DISCUSSION--Immunohistochemical studies are of value in the diagnosis of TA-GvHD. Aberrant keratinocyte expression of HLA-DR and dermal and epidermal infiltration of CD4+ T cells are immunopathological features of TA-GvHD. Immunohistochemical analysis of skin biopsy tissue using antibodies to these markers is thus a useful investigation in pancytopenic patients presenting with unexplained rashes.