Liver dysfunction is common in allogeneic bone marrow graft recipients, but no systematic studies of pre- and posttransplantation liver biopsies have been performed to identify and compare hepatic lesions. This study involved 25 consecutive patients who had undergone serial viral screening tests, liver tests, and pre- and posttransplantation liver biopsy. The aims were to ascertain the origin of liver disorders prior to bone marrow transplantation, to determine the mechanism and severity of liver dysfunction occurring early after transplantation, and to identify a possible relationship between pre-existing liver lesions and the frequency and nature of early liver dysfunction after transplantation. Pretransplantation biochemical liver tests were abnormal in 72% of patients, despite the absence of clinical liver disease. Eleven patients had chronic viral hepatitis B or C. Mild or moderate histological lesions were present in all the patients, with bile duct abnormalities in 48%, central vein abnormalities in 24%, sinusoidal fibrosis in 52%, portal fibrosis in 88%, portal necrosis in 52%, and parenchymal siderosis in 76%. After transplantation, fatal veno-occlusive disease occurred in two patients and biochemical abnormalities occurred in 24. Coded review of needle biopsy specimens failed to provide a single diagnosis. Histological lesions differed between pre- and posttransplantation biopsy specimens only by increased iron overload (96%, P<0.01). We conclude that pretransplant liver lesions contribute to hepatic dysfunction early after bone marrow transplantation, being very similar in nature and degree to lesions observed posttransplantation.