BACKGROUND: Previous studies demonstrated that liver function in a veno-occlusive region is approximately 40% of that in a non-veno-occlusive region after hepatectomy with excision of major hepatic vein. We validated the preoperative assessment of future remnant liver (FRL) function based on 40% decreased function of the veno-occlusive region. METHODS: Sixty patients who underwent hepatectomy with excision of major hepatic vein were analyzed. The FRL functions of the veno-occlusive and non-veno-occlusive regions were calculated with 99mTc-galactosyl human serum albumin scintigraphy single-proton emission computed tomography fusion system and SYNAPSE VINCENT® preoperatively. Risk assessment for hepatectomy was evaluated based on indocyanine green retention at 15 min, and patients with insufficient FRL function were described as marginal. RESULTS: The median volume and function of the veno-occlusive region per whole liver were 111 ml and 11.0%, respectively. When the function of the veno-occlusive region was presumed as 0%, 40%, and 100%, the FRL function was 62.5%, 68.4%, and 75.0% and 21, 15, and 7 patients were classified as marginal, respectively. When the function of the veno-occlusive region was presumed as 40%, the posthepatectomy liver failure (PHLF) rate of marginal patients was significantly higher than that of safe patients (46.7% vs 8.9%, P = 0.002). Multivariable analysis indicated that marginal FRL function based on 40% decreased function of the veno-occlusive region was the only independent risk factor for PHLF (odds ratio 8.97, P = 0.002) after extended hepatectomy. CONCLUSION: Assessment of preoperative FRL function based on 40% decreased function of the veno-occlusive region may have high validity.