The purpose of this study was to assess factors influencing the end-tidal concentrations of isoflurane within a bispectral index (BIS) range of 45-55 among healthy live liver donors (n = 11), chronic hepatitis B patients undergoing hepatectomy hepatocellular carcinoma (n = 10), and end-stage liver disease patients undergoing liver transplantation (n = 7). Patients data collected prospectively were compared among the groups using one-way analysis of variance as well as univariate and multivariate techniques. The results showed that end-stage liver disease patients required the least end-tidal isoflurane concentration. Patients with hepatocellular carcinoma with cirrhosis required intermediate end-tidal isoflurane concentrations; healthy live liver donors required the highest end-tidal isoflurane concentrations to provide sufficient anesthetic depth, as monitored by a target BIS (range, 45-55). Upon multivariate analysis, liver function was the only significant factor influencing the likelihood of lowering the end-tidal isoflurane concentration by 4 hours after anesthesia induction (P = .026). In conclusion, we recommend a preset target BIS within the range of 45-55 to monitor the depth of anesthesia during partial hepatectomy and liver transplantation because end-tidal isoflurane concentration requirements are different for patients with various liver status. This strategy may protect the patients from intraoperative recall or anesthesia over-depth as a consequence of insufficient or overdose of anesthesia, respectively.