Ketogenesis was evaluated in 33 critically ill hepatectomized patients in relation to the arterial ketone body ratio (acetoacetate to 3-hydroxybutyrate), which reflects hepatic mitochondrial redox state. In 15 patients whose arterial ketone body ratio decreased to below 0.4, blood ketone body levels were significantly increased concomitant with marked increase of blood lactate and plasma alanine levels. In the 6 survivors of these 15 patients, the arterial ketone body ratio was restored within the next 2 days, and blood ketone body levels were decreased. By contrast, in the nine non-survivors, the arterial ketone body ratio remained below 0.4, and blood ketone body levels were decreased, accompanied by significant increases in blood lactate and plasma alanine levels in the terminal stages. These results suggest that ketogenesis acts as an alternative process for ATP synthesis in the liver in critically ill patients. Death occurs when the liver falls into an energy crisis concomitant with the cessation of ketogenesis.