The redox tolerance test introduced in this article attempts to quantify the deterioration of hepatic mitochondrial energy metabolism by measuring the changes in arterial ketone body ratio in response to 75-g oral glucose loading, and is discussed in relation to its predictive value for assessing surgical risk in hepatectomy. The indicator, called redox tolerance index (RTI), represents a 100-fold cumulative enhancement of ketone body ratio relative to glucose level (100 x delta KBR/delta glucose). The redox tolerance index was significantly different between 31 liver cirrhotics and 10 normal volunteers (p less than 0.001). Subjects were divided into three classes (I: RTI greater than or equal to 1.0, II: 0.5 less than or equal to RTI less than 1.0, III: RTI less than 0.5). Postoperative mortality was significantly different among the three classes in 127 hepatic resections (chi 2 = 9.843, p less than 0.01). Of 97 hepatocellular carcinoma cases, major hepatic resections in class III showed significantly higher postoperative morbidity and mortality rates (p less than 0.05 and p less than 0.05, respectively). The present findings indicate that RTI based on redox theory is of potential value in predicting posthepatectomy outcome.