The majority of patients with acute liver failure (ALF) die waiting for orthotopic liver transplantation (OLT). No other treatment modality is shown to improve survival. This study was conducted to assess the safety and feasibility of hepatocyte transplantation (HT) and subsequent engraftment and function of donor cells. Functional and structural integrity of cryopreserved and thawed human hepatocytes were assessed by their morphological characteristics, induction of P-4501A1 transcription, and survival in vivo by xenotransplantation into rats. Five patients with severe ALF underwent intrasplenic (4 patients) and/or intrahepatic (2 patients) HT through angiography under cyclosporine immunosuppression. All patients had grade III to IV encephalopathy and factor V levels less than 0.5 U/mL, were ventilator and dialysis dependent, and were not OLT candidates. Three of the 5 patients who survived 48 hours after HT had substantial improvement in encephalopathy scores, arterial ammonia levels, and prothrombin times. Clinical improvement was paralleled by an increase in aminopyrine and caffeine clearances. All 3 patients lived substantially longer than expected based on clinical experience after HT (12, 28, and 52 days) but eventually died. Postmortem examination showed the presence of transplanted hepatocytes in liver and spleen by light microscopy and fluorescent in situ hybridization (FISH). Cryopreserved and thawed human hepatocytes can be transplanted into recipients with ALF with some acceptable but definite complications. Engraftment of donor hepatocytes was proven by histological examination and FISH by both transjugular biopsy and at autopsy. Improvement in brain edema, encephalopathy grade, and clearance of antipyrine and caffeine suggested function, albeit with a 24to 72-hour delay posttransplantation.