BACKGROUND AND AIMS: Lenvatinib, an oral molecular target drug used for the treatment of hepatocellular carcinoma (HCC), has a high protein binding rate, resulting in high variability of free drug concentration in blood depending on patient conditions and drug interactions. In this study, an ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS) assay was established to measure free lenvatinib concentrations. The novel assay was used to measure free lenvatinib concentrations in plasma of HCC patients, and the effect of hepatic dysfunction on free lenvatinib concentrations was evaluated. MATERIALS AND METHODS: We studied 31 HCC patients treated orally with lenvatinib at Oita University Hospital. Blood samples were collected at seven time points on the first day of lenvatinib administration to measure total and free lenvatinib concentrations using UHPLC-MS/MS. RESULTS: Pharmacokinetic (PK) parameters were calculated, and hepatic function was assessed using Child-Pugh (CP) classification and CP score. Patients were divided into CP class A (n = 22) and class B (n = 9), and PK parameters were compared between the two groups. The CP class B group had significantly higher trough concentrations (C CONCLUSIONS: The results of this study indicate that free lenvatinib concentration may reflect the effects of declining hepatic function more sensitively than total lenvatinib concentration. Therefore, measurement of free lenvatinib concentration may be clinically useful in patients with impaired hepatic function.