In boron neutron capture therapy (BNCT), B-10-4-borono-L-phenylalanine (BPA) is commonly used as a B-10 carrier. PET using 4-borono-2-F-18-fluoro-phenylalanine (F-18-FBPA PET) has been performed to estimate boron concentration and predict the therapeutic effects of BNCT; however, the association between tumor uptake of F-18-FBPA and boron concentration in tumors remains unclear. The present study investigated the transport mechanism of F-18-FBPA and BPA, and evaluated the utility of F-18-FBPA PET in predicting boron concentration in tumors. The transporter assay revealed that 2-aminobicyclo-(2.2.1)-heptane-2-carboxylic acid, an inhibitor of the L-type amino acid transporter, significantly inhibited F-18-FBPA and C-14-4-borono-L-phenylalanine (C-14-BPA) uptake in FaDu and LN-229 human cancer cells. F-18-FBPA uptake strongly correlated with C-14-BPA uptake in 7 human tumor cell lines (r = .93; P < .01). PET experiments demonstrated that tumor uptake of F-18-FBPA was independent of the administration method, and uptake of F-18-FBPA by bolus injection correlated well with BPA uptake by continuous intravenous infusion. The results of this study revealed that evaluating tumor uptake of F-18-FBPA by PET was useful for estimating B-10 concentration in tumors.