Postmortem and surgical specimens of liver from 20 patients who had undergone allogeneic bone marrow transplantation for a variety of disorders were examined. The lesions fell into five major categories: bile duct atypia often associated with portal tract fibrosis (8 cases), veno-occlusive disease (2 cases), small foci of non-zonal hepatocyte necrosis (3 cases), opportunistic infections (3 cases), and a miscellaneous group of non-specific abnormalities. Our findings, in conjunction with those in experimental animals, point strongly to the bile duct lesion being a specific manifestation of graft versus host disease (GvHD). Veno-occlusive disease has also been reported recently as a possible manifestation of a graft versus host reaction and, although both our patients with this lesion had evidence of GvHD in the skin and gastrointestinal tract, both had also received irradiation and 6-thioguanine, and these may have been responsible. The foci of hepatocyte necrosis could not be attributed to GvHD with any confidence as the lesion was of only minor severity, infrequent, highly non-specific in appearance and, furthermore, did not correlate well with the presence of bile duct lesions. The absence of specific clinical and biochemical findings in human hepatic GvHD stresses the need for biopsy to make a firm diagnosis. However, the patchy distribution of the bile duct lesion and its absence from the needle biopsies examined in this study suggest possible sampling problems, and further study is necessary to assess its value.