Acute graft-versus-host disease (aGVHD) of the liver was studied with fine-needle aspiration biopsies of thirty-four bone marrow transplant recipients. White cell differentials of liver FNABs and simultaneously taken blood samples were performed, and the increment and corrected increment methods were used to quantitate the inflammatory reaction in the liver. Biopsies taken before transplantation were used as the baseline. During aGVHD, the percentage of lymphoid cells and monocytes increased in the liver. The appearance of immunological blasts, together with a high proportion of activated lymphocytes in the FNABs, were typical findings during aGVHD. In patients with apparent prolonged liver graft-versus-host disease small lymphocytes were the predominating cell type. After initiating corticosteroid treatment, the number of blasts and the proportion of activated lymphocytes decreased. There was no significant difference in the proportions of CD4- and CD8-positive lymphoid cells in FNABs during or after aGVHD.