Abstract: Hepatocyte regeneration is essential for recovery in acute hepatic failure, although it requires a large amount of energy. The ratio of acetoacetate to beta-hydroxybutyrate in arterial blood has been reported to reflect the cellular energy charge of hepatocytes, and we proposed that the recovery of the ratio in the early days of acute hepatic failure is essential for survival. However, there is no report on any marker of regeneration to confirm this hypothesis. In this study, we have assessed this ratio and the serum α-fetoprotein level sequentially in 26 patients with acute hepatic failure. Ten patients recovered and 16 died. The arterial blood ketone body ratio 3 days after the onset of hepatic encephalopathy of grade II or more was below 0.6 in 15 of the 16 nonsurvivors, whereas that in the 10 survivors was above 0.6. There was a positive correlation between the arterial blood ketone body ratio and the maximal concentration of α-fetoprotein (r=0.465, p<0.02 by Student's t-test). These data indicate that the arterial blood ketone body ratio is a marker for the capacity of the liver to regenerate and for the prognosis in patients with acute hepatic failure: effective hepatocyte regeneration may be impossible if these metabolic changes in acute hepatic failure impair the hepatocyte energy charge severely.