ABSTRACT— Encephalopathy and severe coagulopathy in patients with acute hepatitis (AH) are good markers for the diagnosis of fulminant hepatitis (FH), which occurs in only about 1% of AH patients. However, even if patients show severe coagulopathy, it is quite difficult to predict FH before the onset of encephalopathy. The ratio of acetoacetate/beta-hydroxybutyrate in arterial blood (KBR) has been reported to reflect the cellular energy charge level in hepatocytes. In our previous report, KBR was quite low in FH patients and was an excellent marker for predicting the prognosis. KBR of normal subjects is distributed in a range of 1.0–2.1 (1.54 ± 0.26, mean ±SD). In this study, we assessed KBR serially in 15 AH patients with severe coagulopathy (hepaplastin test (HPT) < 40%), including seven patients who developed FH, to see if we could predict FH by using KBR as a marker. Seven patients with KBR < 0.6 of long duration (4 days or more) were complicated with hepatic encephalopathy (HE) and it took 3 or more days of KBR below 0.6 before HE appeared. The other eight patients with KBR < 0.6 of short duration (less than 4 days) were not complicated with HE. These data suggest that AH patients with HPT < 40% and a 3-day duration of KBR < 0.6 are at serious risk of FH.