PURPOSE: The early diagnosis of primary hepatocellular carcinoma (HCC) has the potential to lead to significant improvements for the treatment and survival rates of cancer patients. 2-Deoxy-2-[F]fluoro-D-glucose ([F]FDG) is often used as a tracer for positive emission tomography (PET) and X-ray computed tomography (CT) imaging of cancer cells; however, [F]FDG PET/CT cannot currently be used as an early diagnostic technique for HCC. This is because these cancer cells express high levels of glucose-6-phosphatase (G6Pase) that is responsible for poor cellular retention of [F]FDG. Here, we sought to investigate the feasibility of metformin treatment to promote [F]FDG uptake in HCC and the mechanism involved. PROCEDURES: Human SMMC-7721 HCC cells were treated with metformin (up to 10 mM) or FoxO1 siRNA. The transcriptional and expression levels of FoxO1 and G6Pase were determined by quantitative RT-PCR and Western blotting, respectively. The feasibility of using metformin to promote [F]FDG uptake was investigated by both in vitro cell uptake analysis and in vivo microPET/CT imaging. Stable doxycycline-inducible cell lines with FoxO1-overexpression (FoxO1-OE) and FoxO1-knockdown (FoxO1-KD) were constructed to evaluate the impact of FoxO1 on G6Pase expression in vitro and [F]FDG uptake in vivo. RESULTS: Treatment of HCC cells with metformin (Met) leads to a dose-dependent reduction in the expression levels of FoxO1 at the protein level, but not at the mRNA level. Met-induced phosphorylation of FoxO1 initiates a reduction in the expression levels of G6Pase mRNA, which results in an overall increase in the uptake of [F]FDG into HCC cells and tumors. CONCLUSIONS: We propose that treatment of HCC cells with Met may be a useful strategy for improving the efficacy of [F]FDG as a tracer for PET/CT imaging of HCC tumors in patients.