Acute liver failure (ALF) remains a serious problem in critical care with a high rate of mortality. Although the pathophysiology of ALF has not been fully elucidated, oxidative stress has been in part implicated in its pathogenesis. Heme oxygenase-1 (HO-1) is known to be induced not only by its substrate, heme, but also by various oxidative stresses, and thought to play an important role in the protection of the host from oxidative tissue injuries. In the present study, we examined expression of HO-1 as well as the non-specific delta-aminolevulinate synthase (ALAS-N, or ALAS1), the rate-limiting enzyme in heme catabolism and biosynthesis, respectively, in the livers of patients with ALF. Compared with livers from control subjects who had various disorders, but normal hepatic function, HO-1 in the liver of ALF patients was highly up-regulated at both transcriptional and protein levels. Immunohistochemical studies demonstrated that HO-1 expression occurred predominantly in hepatocytes, but not in non-parenchymal cells. In contrast to HO-1, ALAS1 gene expression was markedly down-regulated in ALF patients compared with controls. These findings suggest that, in the liver of ALF patients, there may be an increase in free heme concentration which up-regulates HO-1 gene expression, while down-regulating ALAS1 gene expression, resulting in markedly altered heme metabolism and liver function.