To evaluate residual liver functional reserve before major hepatectomy, volumetry of the residual liver was measured by computed tomography (CT) and the uptake of l-[methyl- 11 C] methionine of the residual liver was expressed by differential absorption ratio (DAR) with positron emission tomography (PET). Residual liver functional volume was quantified as the functional volume index (FVI). FVI of 11 normal whole liver was 8697 ± 2009 (mean S.D.). Cases, 11, of malignant liver tumors with obstructive jaundice underwent major hepatectomy. FVI of the eight patients (5561 ± 1087) who had good course after operation was significantly higher than that of the other cases (266 +- 685) died of liver failure (P < 0.005). FVI is a useful index to quantify residual liver functional reserve to avoid liver failure after major hepatectomy.