Although the mechanism involved in acute liver failure (ALF) has not yet been clarified, microcirculatory disturbance in the liver appears to play a pivotal role in the progression of this disease. To confirm the existence of hepatic hypoxic conditions, we evaluated the amounts of lactate dehydrogenase (LDH) in hepatocytes, since its production increases under low oxygen concentrations. Histological examination was performed in 7 patients with ALF. All 7 patients underwent a liver biopsy during the acute phase of ALF, and 4 of them underwent a second biopsy during the recovery phase. The obtained samples were immunohistochemically stained with anti-LDH5 and anti-CD-68 antibodies. As controls, we examined samples from patients with acute hepatitis, chronic hepatitis and liver cirrhosis. The production of LDH by hepatocytes and the number of CD-68 positive macrophages were markedly increased at the acute phase of ALF, and both of these effects abruptly decreased during the recovery phase. By contrast, most of the samples from the patients with chronic hepatitis and acute hepatitis showed slightly any increase in LDH staining. In cirrhotic patients, partially elevated LDH production was observed mainly around the central vein, but the staining intensity was less compared to that in ALF patients. Our findings indicate that hepatic hypoxic conditions exist in ALP at the acute phase and seem to closely correlate with macrophage overactivation in the liver. We speculate that microcirculatory disturbance may be a key process in the development and progression of ALF.